BALANCE (derived through the Fr. from the Late Lat. _bilantia_, an apparatus for weighing, from _bi_, two, and _lanx_, a dish or scale), a term originally used for the ordinary beam balance or weighing machine with two scale pans, but extended to include (with or without adjectival qualification) other apparatus for measuring and comparing weights and forces. In addition to beam and spring balances (see WEIGHING MACHINES), apparatus termed "torsion balances," in which forces are measured or compared by their twisting moment on a wire, are used, especially in gravitational, electrostatic and magnetic experiments (see GRAVITATION and ELECTROMETER). The term also connotes the idea of equality or equalization; _e.g._ in the following expressions: "balance," in bookkeeping, the amount which equalizes the debit and credit accounts; "balance wheel," [v.03 p.0235] in horology, a device for equalizing the relaxing of a watch or clock spring (see CLOCK); the "balancing of engines," the art of minimizing the total vibrations of engines when running, and consisting generally in the introduction of ma.s.ses which induce vibrations opposed to the vibrations of the essential parts of the engine.

BALANCE OF POWER, a phrase in international law for such a "just equilibrium" between the members of the family of nations as should prevent any one of them from becoming sufficiently strong to enforce its will upon the rest. The principle involved in this, as Hume pointed out in his _Essay on the Balance of Power_, is as old as history, and was perfectly familiar to the ancients both as political theorists and as practical statesmen. In its essence it is no more than a precept of commonsense born of experience and the instinct of self-preservation; for, as Polybius very clearly puts it (lib. i. cap. 83): "Nor is such a principle to be despised, nor should so great a power be allowed to any one as to make it impossible for you afterwards to dispute with him on equal terms concerning your manifest rights." It was not, however, till the beginning of the 17th century, when the science of international law took shape at the hands of Grotius and his successors, that the theory of the balance of power was formulated as a fundamental principle of diplomacy. According to this the European states formed a sort of federal community, the fundamental condition of which was the preservation of the balance of power, _i.e._ such a disposition of things that no one state or potentate should be able absolutely to predominate and prescribe laws to the rest; and, since all were equally interested in this settlement, it was held to be the interest, the right and the duty of every power to interfere, even by force of arms, when any of the conditions of this settlement were infringed or a.s.sailed by any other member of the community.[1] This principle, once formulated, became an axiom of political science. It was impressed as such by Fenelon, in his _Instructions_, on the young duke of Burgundy; it was proclaimed to the world by Frederick the Great in his _Anti-Machiavel_; it was re-stated with admirable clearness in 1806 by Friedrich von Gentz in his _Fragments on the Balance of Power_. It formed the basis of the coalitions against Louis XIV.

and Napoleon, and the occasion, or the excuse, for most of the wars which desolated Europe between the congress of Munster in 1648 and that of Vienna in 1814. During the greater part of the 19th century it was obscured by the series of national upheavals which have remodelled the map of Europe; yet it underlay all the efforts of diplomacy to stay or to direct the elemental forces let loose by the Revolution, and with the restoration of comparative calm it has once more emerged as the motive for the various political alliances of which the ostensible object is the preservation of peace (see EUROPE: _History_).

An equilibrium between the various powers which form the family of nations is, in fact,--as Professor L. Oppenheim (_Internat. Law_, i. 73) justly points out--essential to the very existence of any international law. In the absence of any central authority, the only sanction behind the code of rules established by custom or defined in treaties, known as "international law," is the capacity of the powers to hold each other in check. Were this to fail, nothing could prevent any state sufficiently powerful from ignoring the law and acting solely according to its convenience and its interests.

See, besides the works quoted in the article, the standard books on International Law (_q.v._).

(W. A. P.)

[1] Emerich de Vattel, _Le Droit des gens_ (Leiden, 1758).

BALANCE OF TRADE, a term in economics belonging originally to the period when the "mercantile theory" prevailed, but still in use, though not quite perhaps in the same way as at its origin. The "balance of trade" was then identified with the sum of the precious metals which a country received in the course of its trading with other countries or with particular countries. There was no doubt an idea that somehow or other the amount of the precious metals received represented profit on the trading, and each country desired as much profit as possible. Princes and sovereigns, however, with political aims in view, were not close students of mercantile profits, and would probably have urged the acquisition of the precious metals as an object of trade even if they had realized that the country as a whole was exporting "money's worth" in order to buy the precious metals which were desired for political objects. The "mercantile theory" was exploded by Adam Smith's demonstration that gold and silver were only commodities like others with no special virtue in them, and that they would come into a country when there was a demand for them, according to the amount, in proportion to other demands, which the country could afford to pay; but the ideas in which the theory itself has originated have not died out, and the idea especially of a "balance of trade" to which the rulers of a country should give attention is to be found in popular discussions of business topics and in politics, the general notion being that a nation is prosperous when its statistics show a "trade balance" in its favour and unprosperous when the reverse is shown. In modern times the excess of imports over exports or of exports over imports, shown in the statistics of foreign trade, has also come to be identified in popular speech with the "balance of trade," and many minds are no doubt imbued with the ideas (1) that an excess of imports over exports is bad, and (2) an excess of exports over imports is the reverse, because the former indicates an "unfavourable"

and the latter a "favourable" trade balance. In the former case it is urged that a nation so circ.u.mstanced is living on its capital. Exact remedies are not suggested, although the idea of preventing or hampering foreign imports as a means of developing home trade and of thus altering the supposed disastrous trade balance is obviously the logical inference from the arguments. A consideration of these ideas and of recent discussions about imports and exports, appears accordingly to be needed, although the "mercantile theory" is itself exploded.

The phrase "balance of trade," then, appears to be an application of a trader's language in his own business to the larger affairs of nations or rather of the aggregate of individuals in a nation engaged in foreign trade. A trader in his own books sets his sales against his purchases, and the amount by which the former exceed the latter is his trade balance or profit. What is true of the individual, it is a.s.sumed, must be true of a nation or of the aggregate of individual traders in a nation engaged in the foreign trade. If their collective sales amount to more than their collective purchases the trade balance will be in their favour, and they will have money to receive. Contrariwise, if their purchases amount to more than their sales, they will have to pay money, and they will presumably be living on their capital. The argument fails, however, in many ways. Even as regards the experience of the individual trader, it is to be observed that he may or may not receive his profit, if any, in money. As a rule he does not do so. As the profit accrues he may invest it either by employing labour to add to his machinery or warehouses, or by increasing his stock-in-trade, or by adding to his book debts, or by a purchase of stocks or shares outside his regular business. At the end of a given period he may or may not have an increased cash balance to show as the result of his profitable trading. Even if he has an increased cash balance, according to the modern system of business, this might be a balance at his bankers', and they in turn may have invested the amount so that there is no stock of the precious metals, of "hard money," anywhere to represent it. And the argument fails still further when applied to the transactions between nations, or rather, to use the phrase already employed, between the aggregate of individuals in nations engaged in the foreign trade. It is quite clear that if a nation, or the individuals of a nation, do make profit in their foreign trading, the amount may be invested as it accrues--in machinery, or warehouses, or stock-in-trade, or book debts, or stocks and shares purchased abroad, so that there may be no corresponding "balance of trade" to bring home. There is no doubt also that what may be is in reality what largely happens. A prosperous foreign trade carried on by any country implies a continuous investment by that country either abroad or at home, and there may or may not be a balance receivable in actual gold and silver.

[v.03 p.0236] In another particular the argument also fails. In the aggregate of individual trading with various countries, there may sometimes be purchases and sales as far as the individuals are concerned, but not purchases and sales as between the nations. For example, goods are exported from the United Kingdom, ammunition and stores and ships, which appear in the British returns as exports, and which have really been sold by individual British traders to individuals abroad; but these sales are not set off by any purchases on the other side which come into the international account, as the set-off is a loan by the people of one country to the people or government of another. The same with the export of railway and other material when goods are exported for the purpose of constructing railways or other works abroad. The sales are made by individuals in the United Kingdom to individuals abroad; but there is no set-off of purchases on the other side. _Mutatis mutandis_ the same explanation applies to the remittance of goods by one country to another, or by individuals in one country to individuals in another to pay the interest or repay the capital of loans which have been received in former times. These are all cases of the movement of goods irrespective of international sales and purchases, though the movements themselves appear in the international records of imports and exports, and therefore it seems to be a.s.sumed, though without any warrant, in the international records of the balance of trade. There is yet another failure in the comparison. The individual trader would include in his sales and purchases services such as repairs performed by him for others, and similar services which others do for himself; but no similar accounts are kept of the corresponding portions of international trade such as the earning of freights and commissions, although in strictness, it is obvious, they belong as much to international trade as the imports and exports themselves, which cannot therefore show a complete "balance of trade."

The illusions which may result then from the confusion of ideas between a balance of trade or profit, and a balance of cash paid or received, and from the identification of an excess of imports over exports or of exports over imports with the balance of trade itself, though they are not the same things, hardly need description. The believers in such illusions are not ent.i.tled to any hearing as economists, however, much they may be accepted in the market-place or among politicians.

The "balance of trade" and "the excess of imports over exports" are thus simply pitfalls for the amateur and the unwary. On the statistical side, moreover, there is a good deal more to be urged in order to impress the student with care and attention. The records of imports and exports themselves may vary from the actual facts of international purchases and sales. The actual values of the goods imported and paid for by the nation may vary from the published returns of imports, which are, by the necessity of the case, only estimated values. And so with the exports. The actual purchases and sales may be something very different. A so-called sale may prove abortive through its not being paid for at all, the debtor failing altogether. In any case the purchases of a year may not be paid for by the sales of the year, and the "squaring" of the account may take a long time.

Still more the estimates of value may be so taken as not to give even an approximately correct account as far as the records go. Thus in the plan followed in the United Kingdom imports are valued as at the port where they arrive and exports at the port where they are despatched from--a plan which so far places them on an equal footing for the purpose of striking a balance of trade. But in the import and export records of the United States a different plan is followed. The imports are no longer valued as at the port of arrival with the freight and other charges included, but as at the port of shipment. The results on the balance of trade drawn out must accordingly be quite different in the two cases. With other countries similar differences arise. To deduce then from records of imports and exports any conclusions as to the excess of imports or exports at different times is a work of enormous statistical difficulty. Excellent ill.u.s.trations will be found in J. Holt Schooling's _British Trade Book_ (1908).

The country which presents the most interesting questions in connexion with the study is the United Kingdom, with its largely preponderating foreign trade. Its annual imports and exports, excluding bullion, exceed 800 millions sterling, and the bullion one year with another is 100 millions more. Its excess of imports, moreover, between the middle and end of the 19th century gradually rose from a small figure to 180 millions sterling annually, and occasioned the popular discussion referred to respecting an "adverse" balance of trade, and particularly the belief existing in many quarters that the nation is living on its capital. The result has been a new investigation of the subject, so as to bring out and present the credits to which the country is ent.i.tled in its trade as a shipowner and commission merchant, and to exhibit at the same time the magnitude of British foreign investments, which cannot be less than 2000 millions sterling and must bring in an enormous annual income. Other countries such as France, Germany, Belgium, Holland, Denmark, Norway and Sweden, are in the same condition, though their foreign trade is not on the same scale, and similar rules apply to the reading of their import and export accounts.

The United States is a conspicuous instance of a country which in the first decade of the 20th century was still in the position of a borrower and had a large excess of exports, though there were signs of a change in the opposite direction. New countries generally, such as Canada, Australia and the South American countries, resemble the United States. Comparisons are made difficult by the want of uniformity in the methods of stating the figures, but that different countries have to be grouped according as they are indebted or creditor countries is undeniable, and no study of the trade statistics is possible without recognition of the underlying economic circ.u.mstances.

In conclusion it may be useful to repeat the main propositions laid down as to the balance of trade, (1) A "balance of trade" to the individual trader, from whose experience the phrase comes, is not necessarily, as is supposed, a balance received or receivable in the precious metals. It may be invested as it accrues--in machinery, or warehouses, or stock-in-trade, or in book debts, or in stocks and shares or other property outside the trader's business, as well as in cash. (2) What is true of the individual trader is also true of the aggregate of individuals engaged in the foreign trade of a country. Cash is only one of the forms in which they may elect to be paid.

(3) The imports and exports recorded in the statistical returns of a country do not correspond with the purchases and sales of individual traders, as the sales especially may be set off by loans, while the so-called imports may include remittances of interest and of capital repaid. (4) When capital is repaid the country receiving it need not be living on it, but may be investing it at home. (5) The foreign trading of countries may also comprise many transactions, such as the earning of freights and commissions, which ought to appear in a proper account showing a balance of trade, as similar transactions appear in an individual trader's account, but which are not treated as imports or exports in the statistical returns of a nation's foreign trade. (6) Import and export returns themselves are not the same as accounts of purchases and sales; the values are only estimates, and must not be relied on literally without study of the actual facts. (7) Import and export returns in different countries are not in all cases taken at the same point, there being important variations, for instance, in this respect between the returns of two great countries, the United Kingdom and the United States, which are often compared, but are really most difficult to compare. (8) The United Kingdom is a conspicuous instance of a country which has a great excess of imports over exports in consequence of its large lending abroad in former times; while its accounts are specially affected by the magnitude of its services as a trading nation carrying pa.s.sengers and goods all over the world, which do not result, however, in so-called "exports." The United States, on the other hand, is a conspicuous instance of an indebted nation, which has or had until lately few or no sums to its credit in foreign trade except the visible exports. (9) The various countries of the world naturally fall into groups. The nations of western Europe, such as France, Germany, Belgium, Holland, Sweden and Norway, fall into a [v.03 p.0237]

group with Great Britain as creditor nations, while Canada, Australasia and the South American countries fall into a group with the United States as undeveloped and indebted countries, So also of other countries, each belongs naturally to one group or another. (10) The excess of imports or exports may vary indefinitely at different times according as a creditor country is receiving or lending at the time, or according as a debtor country is borrowing or paying off its debts at the time, but the permanent characteristics are always to be considered.

(R. GN.)

[Ill.u.s.tration: FIG. 1.--_Ptychodera flava_ (New Caledonia), from above; about life size.]

BALANOGLOSSUS, the general name given to certain peculiar, opaque, worm-like animals which live an obscure life under stones, and burrow in the sand from between tide-marks down to the abyssal regions of the sea.

Their colour is usually some tone of yellow with dashes of red, brown and green, and they frequently emit a pungent odour. The name has reference to the tongue-shaped muscular proboscis by which the animal works its way through the sand. The proboscis is not the only organ of locomotion, being a.s.sisted by the succeeding segment of the body, the buccal segment or collar. By the waves of contraction executed by the proboscis accompanied by inflation of the collar, progression is effected, sometimes with marvellous rapidity. The third body region or trunk may attain a great length, one or two feet, or even more, and is also muscular, but the truncal muscles are of subordinate importance in locomotion, serving princ.i.p.ally to promote the peristaltic contractions of the body by which the food is carried through the gut. The function of alimentation is closely a.s.sociated with that of locomotion, somewhat as in the burrowing earthworm; in the excavation of its burrows the sand is pa.s.sed through the body, and any nutrient matter that may adhere to it is extracted during its pa.s.sage through the intestine, the exhausted sand being finally ejected through the vent at the orifice of the burrow and appearing at low tide as a worm casting. In accordance with this manner of feeding, the mouth is kept permanently open and prevented from collapsing by a pair of skeletal cornua belonging to a sustentacular apparatus (the nuchal skeleton), the body of which lies within the narrow neck of the proboscis; the latter is inserted into the collar and surrounded by the anterior free flap of this segment of the body.

When first discovered by J. F. Eschscholtz at the Marshall Islands in 1825, _Balanoglossus_ was described as a worm-like animal belonging to the Echinoderm order of Holothurians or sea-cuc.u.mbers. In 1865 Kowalevsky discovered that the organs of respiration consist of numerous pairs of gill-slits leading from the digestive ca.n.a.l through the thickness of the body-wall to the exterior. On this account the animal was subsequently placed by Gegenbaur in a special cla.s.s of Vermes, the Enteropneusta. In 1883-1886 Bateson showed by his embryological researches that the Enteropneusta exhibit chordate (vertebrate) affinities in respect of the coelomic, skeletal and nervous systems as well as in regard to the respiratory system, and, further, that the gill-slits are formed upon a plan similar to that of the gill-slits of _Amphioxus_, being subdivided by tongue-bars which depend from the dorsal borders of the slits.

_Coelom and Pore-ca.n.a.ls_.--In correspondence with the tri-regional differentiation of the body in its external configuration, the coelom (body-cavity, perivisceral cavity) is divided into three portions completely separated from one another by septa:--(1) proboscis-coelom, or first body-cavity; (2) the collar-coelom, or second body-cavity; (3) truncal coelom, or third body-cavity. Of these divisions of the coelom the first two communicate with the exterior by means of a pair of ciliated pore-ca.n.a.ls placed at the posterior end of their respective segments. The proboscis-pores are highly variable, and frequently only one is present, that on the left side; sometimes the pore-ca.n.a.ls of the proboscis unite to open by a common median orifice, and sometimes their communication with the proboscis-coelom appears to be occluded, and finally the pore-ca.n.a.ls may be quite vestigial. The collar-pores are remarkable for their constancy; this is probably owing co the fact that they have become adapted to a special function, the inhalation of water to render the collar turgid during progression. There are reasons for supposing that the truncal coelom was at one time provided with pore-ca.n.a.ls, but supposed vestiges of these structures have only been described for one genus, _Spengelia_, in which they lie near the anterior end of the truncal coelom.

_Enteron_.--Not only is the coelom thus subdivided, but the enteron (gut, alimentary ca.n.a.l, digestive tube) itself shows indications of three main subsections in continuity with one another:--(1) proboscis-gut (_Eicheldarm_, stomochord, _vide infra_); (2) collar-gut (buccal cavity, throat); (3) truncal gut extending from the collar to the vent.

_Stomochord_.--The proboscis-gut occurs as an outgrowth from the anterior dorsal wall of the collar-gut, and extends forward into the basal (posterior) region of the proboscis, through the neck into the proboscis-coelom, ending blindly in front. Although an integral portion of the gut, it has ceased to a.s.sist in alimentation, its epithelium undergoes vacuolar differentiation and hypertrophy, and its lumen becomes more or less vestigial. It has, in fact, become metamorphosed into a resistant supporting structure resembling in some respects the notochord of the true Chordata, but probably not directly comparable with the latter structure, being related to it solely by way of subst.i.tution. On account of the presence and mode of origin (from the gut-wall) of this organ Bateson introduced the term hemichorda as a phyletic name for the cla.s.s Enteropneusta. As the proboscis-gut appears to have undoubtedly skeletal properties, and as it also has topographical relations with the mouth, it has been designated in English by the non-committal term stomochord. It is not a simple diverticulum of the collar-gut, but a complex structure possessing paired lateral pouches and a ventral convexity (ventral caec.u.m) which rests in a concavity at the front end of the body of the nuchal skeleton (fig. 3). In some species (_Spengelidae_) there is a long capillary vermiform extension of the stomochord in front. The nuchal skeleton is a non-cellular laminated thickening of bas.e.m.e.nt-membrane underlying that portion of the stomochord which lies between the above-mentioned pouches and the orifice into the throat. At the point where the stomochord opens into the buccal cavity the nuchal skeleton bifurcates, and the two cornua thus produced pa.s.s obliquely backwards and downwards embedded in the wall of the throat, often giving rise to projecting ridges that bound a dorsal groove of the collar-gut which is in continuity with the wall of the stomochord (fig. 3).

_Nervous System_.--At the base of the epidermis (which is in general ciliated) there is over the entire surface of the body a layer of nerve-fibres, occurring immediately outside the bas.e.m.e.nt-membrane which separates the epidermis from the subjacent musculature. The nervous system is thus essentially epidermal in position and diffuse in distribution; but an interesting concentration of nerve-cells and fibres has taken place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores. This is the collar nerve-tube. Sometimes the central ca.n.a.l is wide and uninterrupted between the two neuropores; in other cases it becomes broken up into a large number of small closed medullary cavities, and in others again it is obsolete. In one family, the _Ptychoderidae_, the medullary tube of the collar is connected at intermediate points with the epidermis by means of a variable number of unpaired outgrowths from its dorsal wall, generally containing an axial lumen derived from and in continuity with the central ca.n.a.l. These hollow roots terminate blindly in the dorsal epidermis of the collar, and place the nervous layer of the latter in direct connexion with the fibres of the nerve-tube. The exact significance of these roots is a matter for speculation, but it seems possible that they are epiphysial structures remotely comparable with the epiphysial (pineal) complex of the craniate vertebrates. In accordance with this view there would be also some probability in favour of regarding the collar nerve-tube of the Enteropneusta as the equivalent of the cerebral vesicle only of _Amphioxus_ and the Ascidian tadpole, and also of the primary fore-brain of vertebrates.

Special thickenings of the diffuse nervous layer of the epidermis occur in certain regions and along certain lines. In the neck of the proboscis the fibrous layer is greatly thickened, and other intensifications of this layer occur in the dorsal and ventral middle lines of the trunk extending to the posterior end of the body. The dorsal epidermal nerve-tract is continued in front into the ventral wall of the collar nerve-tube, and at the point of junction there is a circular commissural thickening following the posterior rim of the collar and affording a special connexion between the dorsal and ventral nerve-tracts. From the ventral surface of the collar nerve-tube numerous motor fibres may be seen pa.s.sing to the subjacent musculature. These fibres are not aggregated into roots.

[v.03 p.0238] [Ill.u.s.tration: FIG. 2.--Structure of branchial region.

_bc_, coelom.

_tb_, tongue-bars.

_ds_, mesentery.

_pr_, ridge.

_vv_, vessel.

_gp_, gill-pore.

_dn_, dorsal nerve.

_dv_, vessel.

_oe_, oesophagus.

_vs_, mesentery.

_vn_, ventral nerve. ]

_Gill-slits_.--The possession of gill-slits is as interesting a feature in the organization of _Balanoglossus_ as is the presence of tracheae in _Peripatus_. These gill-slits occupy a variable extent of the anterior portion of the trunk, commencing immediately behind the collar-trunk septum. The branchial bars which const.i.tute the borders of the clefts are of two kinds:--(1) Septal bars between two contiguous clefts, corresponding to the primary bars in _Amphioxus_; (2) Tongue-bars. The chief resemblances between _Balanoglossus_ and _Amphioxus_ in respect of the gill-slits may be stated briefly as follows:--([alpha]) the presence of two kinds of branchial bars in all species and also of small crossbars (synapticula) in many species; ([beta]) numerous gill-slits, from forty to more than a hundred pairs; ([gamma]) the addition of new gill-slits by fresh perforation at the posterior end of the pharynx throughout life. The chief differences are, that (a) the tongue-bar is the essential organ of the gill-slit in _Balanoglossus_, and exceeds the septal bars in bulk, while in _Amphioxus_ the reverse is the case; (b) the tongue-bar contains a large coelomic s.p.a.ce in _Balanoglossus_, but is solid in _Amphioxus_; (c) the skeletal rods in the tongue-bars of _Balanoglossus_ are double; (d) the tongue-bar in _Balanoglossus_ does not fuse with the ventral border of the cleft, but ends freely below, thus producing a continuous U-shaped cleft.

The meaning of this singular contrast between the two animals may be that we have here an instance of an interesting gradation in evolution. From serving primitively as the essential organ of the cleft the tongue-bar may have undergone reduction and modification, becoming a secondary bar in _Amphioxus_, subordinate to the primary bars in size, vascularity and development; finally, in the craniate vertebrates it would then have completed its involution, the suggestion having been made that the tongue-bars are represented by the thymus-primordia.

_Gill-pouches and Gill-pores_.--Only rarely do the gill-slits open freely and directly to the exterior (fig. 1). In most species of _Balanoglossus_ each gill-slit may be said to open into its own atrial chamber or gill-pouch; this in its turn opens to the exterior by a minute gill-pore.

There are, therefore, as many gill-pouches as there are gill-slits and as many gill-pores as pouches. The gill-pores occur on each side of the dorsal aspect of the worm in a longitudinal series at the base of a shallow groove, the branchial groove. The respiratory current of water is therefore conducted to the exterior by different means from that adopted by _Amphioxus_, and this difference is so great that the theory which seeks to explain it has to postulate radical changes of structure, function and topography.

_Excretory and Vascular Systems_.--It seems likely that the coelomic pore-ca.n.a.ls were originally excretory organs, but in the existing Enteropneusta the pore-ca.n.a.ls (especially the collar ca.n.a.ls) have, as we have seen, acquired new functions or become vestigial, and the function of excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom. The vascular system itself is quite peculiar, consisting of lacunae and channels dest.i.tute of endothelium, situated within the thickness of the bas.e.m.e.nt-membrane of the body-wall, of the gut-wall and of the mesenteries. The blood, which is a non-corpuscular fluid, is propelled forwards by the contractile dorsal vessel and collected into the central blood-sinus; this lies over the stomochord, and is surrounded on three sides by a closed vesicle, with contractile walls, called the pericardium (_Herzblase_). By the pulsation of the pericardial vesicle (best observed in the larva) the blood is driven into the glomerulus, from which it issues by efferent vessels which effect a junction with the ventral (sub-intestinal) vessel in the trunk. The vascular system does not readily lend itself to morphological comparison between such widely different animals as _Balanoglossus_ and _Amphioxus_, and the reader is therefore referred to the memoirs cited at the end of this article for further details.

[Ill.u.s.tration: FIG. 3.--Structure of anterior end.

a, Arrow from proboscis-cavity (pc) pa.s.sing to left of pericardium (per) and out through proboscis pore-ca.n.a.l.

b^1, arrow from central ca.n.a.l of neurochord (cnc) pa.s.sed out through anterior neuropore.

b^2, ditto; through posterior neuropore.

c, arrow intended to pa.s.s from 1st gill-pouch through collar pore-ca.n.a.l into collar-coelom (cc).

cts, posterior limit of collar.

dv, dorsal vessel pa.s.sing into central sinus (bs).

ev, efferent vessel pa.s.sing into ventral vessel (vv).

epr, epiphysial tubes.

st, stomochord.

vs, ventral septum of proboscis.

sk, body of nuchal skeleton.

m, mouth.

th, throat.

tb, tongue-bars.

tc, trunk coelom. ]

_Reproductive System_.--The s.e.xes are separate, and when mature are sometimes distinguished by small differences of colour in the genital region. Both male and female gonads consist of more or less lobulated hollow sacs connected with the epidermis by short ducts. In their disposition they are either uniserial, biserial or multiserial. They occur in the branchial region, and also extend to a variable distance behind it.

In exceptional cases they are either confined to the branchial region or excluded from it. When they are arranged in uniserial or biserial rows the genital ducts open into or near the branchial grooves in the region of the pharynx and in a corresponding position in the post-branchial region. An important feature is the occurrence in some species (_Ptychoderidae_) of paired longitudinal pleural or lateral folds of the body which are mobile, and can be approximated at their free edges so as to close in the dorsal surface, embracing both the median dorsal nerve-tract and the branchial grooves with the gill-pores, so as to form a temporary peri-branchial and medullary tube, open behind where the folds cease. On the other hand, they can be spread out horizontally so as to expose their own upper side as well as the dorsal surface of the body (fig. 1). These folds are called the genital pleurae because they contain the bulk of the gonads. Correlated with the presence of the genital pleurae there is a pair of vascular folds of the bas.e.m.e.nt membrane proceeding from the dorsal wall of the gut in the post-branchial portion of the branchio-genital region, and from the dorsal angles made by the pleural folds with the body-wall in the pharyngeal region; they pa.s.s, in their most fully developed condition, to the free border of the genital pleurae. These vascular membranes are called the lateral septa. Since there are many species which do not possess these genital pleurae, the question arises as to whether their presence or their absence is the more primitive condition. Without attempting to answer this question categorically, it may be pointed out that within the limits of the family (_Ptychoderidae_) which is especially characterized by their presence there are some species in [v.03 p.0239] which the genital pleurae are quite obsolete, and yet lateral septa occur (_e.g._ _Ptychodera ruficollis_), seeming to indicate that the pleural folds have in such cases been secondarily suppressed.

_Development_.--The development of _Balanoglossus_ takes place according to two different schemes, known as direct and indirect, correlated with the occurrence in the group of two kinds of ova, large and small. Direct development, in which the adult form is achieved without striking metamorphosis by a gradual succession of stages, seems to be confined to the family _Balanoglossidae_. The remaining two families of Enteropneusta, _Ptychoderidae_ and _Spengelidae_, contain species of which probably all pursue an indirect course of development, culminating in a metamorphosis by which the adult form is attained. In these cases the larva, called _Tornaria_, is pelagic and transparent, and possesses a complicated ciliated seam, the longitudinal ciliated band, often drawn out into convoluted bays and lappets. In addition to this ciliated band the form of the _Tornaria_ is quite characteristic and unlike the adult. The Tornaria larva offers a certain similarity to larvae of Echinoderms (sea-urchins, star-fishes, and sea-cuc.u.mbers), and when first discovered was so described. It is within the bounds of possibility that _Tornaria_ actually does indicate a remote affinity on the part of the Enteropneusta to the Echinoderms, not only on account of its external form, but also by reason of the possession of a dorsal water-pore communicating with the anterior body-cavity. In the direct development Bateson showed that the three divisions of the coelom arise as pouches constricted off from the archenteron or primitive gut, thus resembling the development of the mesoblastic somites of _Amphioxus_. It would appear that while the direct development throws light upon the special plan of organization of the Enteropneusta, the indirect development affords a clue to their possible derivation. However this may be, it is sufficiently remarkable that a small and circ.u.mscribed group like the Enteropneusta, which presents such a comparatively uniform plan of composition and of external form, should follow two such diverse methods of development.

_Distribution_.--Some thirty species of _Balanoglossus_ are known, distributed among all the princ.i.p.al marine provinces from Greenland to New Zealand. The species which occurs in the English Channel is _Ptychodera sarniensis_. The _Ptychoderidae_ and _Spengelidae_ are predominantly tropical and subtropical, while the _Balanoglossidae_ are predominantly arctic and temperate in their distribution. One of the most singular facts concerning the geographical distribution of Enteropneusta has recently been brought to light by Benham, who found a species of _Balanoglossus_, _sensu stricto_, on the coast of New Zealand hardly distinguishable from one occurring off j.a.pan. Finally, _Glandiceps abyssicola_ (_Spengelidae_) was dredged during the "Challenger" expedition in the Atlantic Ocean off the coast of Africa at a depth of 2500 fathoms.

AUTHORITIES.--W. Bateson, "Memoirs on the Direct Development of Balanoglossus," _Quart. Journ. Micr. Sci._ (vols. xxiv.-xxvi., 1884-1886); W. B. Benham, "Balanoglossus otagoensis, n. sp," _Q. J. M. S._ (vol. xlii.