The youngest Nauplius (Figure 28) is immediately followed by forms in which a fold of skin runs across the back behind the third pair of feet, and four pairs of stout processes (rudiments of new limbs) sprout forth on the ventral surface. Within the third pair of feet, powerful mandibles are developed.

In a subsequent moult the new limbs (maxillae, and anterior and intermediate maxillipedes) come into action, and in this way the Nauplius becomes a Zoea (Figure 29), agreeing perfectly with the Zoea of the Crabs in the number of the appendages of the body, although very different in form and mode of locomotion and even in many particulars of internal structure. The chief organs of motion are still the two anterior pairs of feet, which are slender and furnished with long setae; the third pair of feet loses its branches, and becomes converted into mandibles dest.i.tute of palpi. The labrum acquires a spine directed forward and of considerable size, which occurs in all the Zoeae of allied species. The biramose maxillipedes appear to a.s.sist but slightly in locomotion. The forked tail reminds us rather of the forms occurring in the lower Crustacea, especially the Copepoda, than of the spatuliform caudal plate which characterises the Zoeae of Alpheus, Palaemon, Hippolyte, and other Prawns, of the Hermit Crabs, the Tatuira and the Porcellanae. The heart possesses only one pair of fissures, and has no muscles traversing its interior like trabeculae, whilst in other Zoeae two pairs of fissures and an interior apparatus of trabeculae are always distinctly recognisable.

During this Zoeal period the paired eyes, the segments of the middle-body and abdomen, the posterior maxillipedes, the lateral caudal appendages and the stump-like rudiments of the feet of the middle-body are formed (Figure 30). The caudal appendages sprout forth like other limbs freely on the ventral surface, whilst in other Prawns, the Porcellanae, etc., they are produced in the interior of the spatuliform caudal plate.

As the feet of the middle-body come into action, simultaneously with other profound changes, the Zoea pa.s.ses into the Mysis- or Schizopod-form (Figure 31). The antennae cease to serve for locomotion, their place is taken by the thoracic feet, furnished with long setae, and by the long abdomen which just before was laboriously dragged along as a useless burden, but now, with its powerful muscles, jerks the animal through the water in a series of lively jumps. The anterior antennae have lost their long setae, and by the side of the last (fourth) joint, endowed with olfactory filaments, there appears a second branch, which is at first of a single joint. The previously multi-articulate outer branch of the posterior antennae has become a simple lamella, the antennal scale of the Prawn; beside this appears the stump-like rudiment of the flagellum, probably as a new formation, the inner branch disappearing entirely. The five new pairs of feet are biramose, the inner branch short and simple, the outer one longer, annulated at the end, furnished with long setae, and kept, as in Mysis, in constant whirling motion. The heart acquires new fissures, and interior muscular trabeculae.

During the Mysis-period, the auditory organs in the basal joint of the anterior antennae are formed; the inner branches of the first three pairs of feet are developed into chelae and the two hinder pairs into ambulatory feet; palpi sprout from the mandibles, branchiae on the thorax, and natatory feet on the abdomen. The spine on the labrum becomes reduced in size. In this way the animal gradually approaches the Prawn-form, in which the median eye has become indistinct, the spine of the labrum, and the outer branches of the cheliferous and ambulatory feet have been lost, the mandibular palpi and the abdominal feet have acquired distinct joints and setae, and the branchiae come into action.

In another Prawn, the various larval states of which may be easily recognised as belonging to the same series by the presence of a dark-yellow, sharply-defined spot surrounding the median eye, the youngest Zoea (Figure 32), probably produced from the Nauplius, agrees in all essential particulars with the species just described; its further development is, however, very different, especially in that neither the feet of the middle, nor those of the hind-body are formed simultaneously, and that a stage of development comparable to Mysis in the number and structure of the limbs does not occur.

(FIGURE 32. Youngest (observed) Zoea of another Prawn. The minute buds of the third pair of maxillipedes are visible. The formation of the abdominal segments has commenced. Paired eyes still wanting. Magnified 45 diam.)

Traces of the outer maxillipedes make their appearance betimes. Then feet appear upon four segments of the middle-body, and these are biramose on the three anterior segments, and simple, the inner branch being deficient, on the fourth segment. On the inner branches the chelae are developed; the outer branches are lost before an inner branch has made its appearance on the fourth segment (Figure 32). The latter again becomes dest.i.tute of appendages, so that in this case at an early period four, and at a later only three, segments of the middle-body bear limbs.

The fifth segment is still entirely wanting, whilst all the abdominal segments have also acquired limbs, and this one after the other, from before backwards. The adult animal, as shown by the three pairs of chelae, will certainly be very nearly allied to the preceding species.*

(* The oldest observed larvae (see Figure 33) are characterised by the extraordinary length of the flagella of the outer antennae, and in this respect resemble the larva of Sergestes found by Claus near Messina (Zeitschr. fur Wiss. Zool. Bd. 13 Taf 27 Figure 14). This unusual length of the antennae leads to the supposition that they belong to our commonest Prawn, which is very frequently eaten, and is most nearly allied to Peneus setiferus of Florida. Claus's Acanthosoma (l.c. Figure 13) is like the younger Mysis-form of the larva figured by me in the 'Archiv fur Naturgeschichte,' 1836, Taf 2, Figure 18, and which I am inclined to refer to Sicyonia carinata.)

The youngest larva of the Schizopod genus Euphausia observed by Claus, stands very near the youngest Zoea of our Prawns; but whilst its anterior antennae are already biramose, and it therefore appears to be more advanced, it still wants the middle maxillipedes. In it also Claus found the heart furnished with only a single pair of fissures. Do not Nauplius-like states in this case also precede the Zoea?

(FIGURE 33. Older larva produced from the Zoea represented in Figure 32.

The last segment and the last two pairs of feet of the middle-body are wanting. Magnified 20 diam.)

The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements. The development of the embryo commences with the formation of the tail! This makes its appearance as a simple lobe, the dorsal surface of which is turned towards and closely applied to that of the embryo. (The young of other Stalk-eyed Crustacea are, as is well known, bent in the egg in such a manner that the ventral surfaces of the anterior and posterior halves of the body are turned towards each other,--in these, therefore, the dorsal, and in Mysis the ventral surface appears convex.) The tail soon acquires the furcate form with which we made acquaintance in the last Prawn-Zoea described. Then two pairs of thick ensiform appendages make their appearance at the opposite end of the body, and behind these a pair of tubercles which are easily overlooked. These are the antennae and mandibles. The egg-membrane now bursts, before any internal organ, or even any tissue, except the cells of the cutaneous layer, is formed. The young animal might be called a Nauplius; but essentially there is nothing but a rough copy of a Nauplius-skin, almost like a new egg-membrane, within which the Mysis is developed. The ten pairs of appendages of the fore- (maxillae, maxillipedes) and middle-body make their appearance simultaneously, as do the five pairs of abdominal feet at a later period. Soon after the young Mysis casts the Nauplius-envelope it quits the brood-pouch of the mother.* (* Van Beneden, who regards the eye-peduncles as limbs, cannot however avoid remarking upon Mysis: "Ce pedicule n'apparait aucunement comme les autres appendices, et parait avoir une autre valeur morphologique.")

For some time, owing to an undue importance being ascribed to the want of a particular branchial cavity, Mysis, Leucifer, and Phyllosoma were referred to the Stomapoda, which are now again limited, as originally by Latreille, to the Mantis-shrimps (Squilla), the Gla.s.s-shrimps (Erichthus) and their nearest allies. Of the developmental history of these we have hitherto been acquainted with only isolated fragments. The tracing of the development in the egg is rendered difficult by the circ.u.mstance, that the Mantis-shrimps do not, like the Decapoda, carry their sp.a.w.n about with them, but deposit it in the subterranean pa.s.sages inhabited by them in the form of thin, round, yellow plates. The sp.a.w.n is consequently exceedingly difficult to procure, and unfortunately it becomes spoilt in a day when it is removed from its natural hatching place, whilst on the contrary the progress of development may be followed for weeks together in the eggs of a single Crab kept in confinement. The eggs of Squilla, like those removed from the body of the Crab, die because they are deprived of the rapid stream of fresh water which the mother drives through her hole for the purpose of her own respiration.

The accompanying representation of the embryo of Squilla shows that it possesses a long, segmented abdomen without appendages, a bilobate tail, six pairs of limbs, and a short heart; the latter only pulsates weakly and slowly. If it acquires more limbs before exclusion, the youngest larva must stand on the same level as the youngest larva of Euphausia observed by Claus.

(FIGURE 34. Embryo of a Squilla, magnified 45 diam. a. heart.

FIGURE 35. Older larva (Zoea) of a Stomapod, magnified 15 diam.)

Of the two larval forms at present known which are with certainty to be ascribed, if not to Squilla, at least to a Stomapod, I pa.s.s over the younger one* (* 'Archiv fur Naturgeschichte' 1863 Taf 1.) as its limbs cannot be positively interpreted, and will only mention that in it the last three abdominal segments are still dest.i.tute of appendages. The older larva (Figure 35), which resembles the mature Squilla especially in the structure of the great raptorial feet and of the preceding pair, still wants the six pairs of feet following the raptorial feet. The corresponding body-segments are already well developed, an unpaired eye is still present, the anterior antennae are already biramose, whilst the flagellum is wanting in the posterior, and the mandibles are dest.i.tute of palpi; the four anterior abdominal segments bear biramose natatory feet, without branchiae; the fifth abdominal segment has no appendages, and this is also the case with the tail, which still appears as a simple lamina, fringed on the hinder margin with numerous short teeth. It is evident that the larva stands essentially in the grade of Zoea.

CHAPTER 8. DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.

Less varied than that of the Stalk-eyed Crustacea is the mode of development of the Isopoda and Amphipoda, which Leach united in the section Edriophthalma, or Crustacea with sessile eyes.

(FIGURE 36. Embryo of Ligia in the egg, magnified 15 diam. D. yelk; L.

liver.)

The Rock-Slaters (Ligia) may serve as an example of the development of the Isopoda. In these, as in Mysis, the caudal portion of the embryo is bent not downwards, but upwards; as in Mysis also, a larval membrane is first of all formed, within which the Slater is developed. In Mysis this first larval skin may be compared to a Nauplius; in Ligia it appears like a maggot quite dest.i.tute of appendages, but produced into a long simple tail (Figure 37). The egg-membrane is retained longer than in Mysis; it bursts only when the limbs of the young Slater are already partially developed in their full number. The dorsal surface of the Slater is united to the larval skin a little behind the head. At this point, when the union has been dissolved a little before the change of skin, there is a foliaceous appendage, which exists only for a short time, and disappears before the young Slater quits the brood-pouch of the mother.

(FIGURE 37. Maggot-like larva of Ligia, magnified 15 diam. R remains of the egg-membrane. We see on the lower surface, from before backwards:--the anterior and posterior antennae, the mandibles, the anterior and posterior maxillae, maxillipedes, six ambulatory feet, the last segment of the middle-body dest.i.tute of appendages, five abdominal feet, and the caudal feet.)

The young animal, when it begins to take care of itself, resembles the old ones in almost all parts, except one important difference; it possesses only six, instead of seven pairs of ambulatory feet; and the last segment of the middle-body is but slightly developed and dest.i.tute of appendages. It need hardly be mentioned that the s.e.xual peculiarities are not yet developed, and that in the males the hand-like enlargements of the anterior ambulatory feet and the copulatory appendages are still deficient.

(FIGURE 38. Embryo of a Philoscia in the egg, magnified 25 diam.)

To the question, how far the development of Ligia is repeated in the other Isopoda, I can only give an unsatisfactory answer. The curvature of the embryo upwards instead of downwards was met with by me as well as by Rathke in Idothea, and likewise in Ca.s.sidina, Philoscia, Tanais, and the Bopyridae,--indeed, I failed to find it in none of the Isopoda examined for this purpose. In Ca.s.sidina also the first larval skin without appendages is easily detected; it is dest.i.tute of the long tail, but is strongly bent in the egg, as in Ligia, and consequently cannot be mistaken for an "inner egg-membrane." This, however, might happen in Philoscia, in which the larval skin is closely applied to the egg-membrane (Figure 38), and is only to be explained as the larval skin by a reference to Ligia and Ca.s.sidina. The foliaceous appendage on the back has long been known in the young of the common Water Slater (Asellus).* (* Leydig has compared this foliaceous appendage of the Water Slaters with the "green gland" or "sh.e.l.l-gland" of other crustacea, a.s.suming that the green gland has no efferent duct and appealing to the fact that the two organs occur "in the same place."

This interpretation is by no means a happy one. In the first place we may easily ascertain in Leucifer, as was also found to be the case by Claus, that the "green gland" really opens at the end of the process described by Milne-Edwards as a "tubercule auditif" and by Spence Bate as an "olfactory denticle." And, secondly, the position is about as different as it can well be. In the one case a paired gland, opening at the base of the posterior antennae, and therefore on the lower surface of the SECOND segment; in the other an unpaired structure rising in the median line of the back BEHIND THE SEVENTH SEGMENT, ("behind the boundary line of the first thoracic segment," Leydig).) That the last pair of feet of the thorax is wanting in the young of the Wood-lice (Porcellionides, M.-Edw.) and Fish-lice (Cymothoadiens, M.-Edw.) has already been noticed by Milne-Edwards. This applies also to the Box-Slaters (Idothea), to the viviparous Globe-Slaters (Sphaeroma) and Shield-Slaters (Ca.s.sidina), to the Bopyridae (Bopyrus, Entoniscus, Cryptoniscus, n.g.), and to the Cheliferous Slaters (Tanais), and therefore probably to the great majority of the Isopoda. All the other limbs are usually well developed in the young Isopoda. In Tanais alone, all the abdominal feet are wanting (but not those of the tail); they are developed simultaneously with the last pair of feet of the thorax.

(FIGURE 39. Embryo of Cryptoniscus planarioides, magnified 90 diam.

FIGURE 40. Last foot of the middle-body of the larva of Entoniscus Porcellanae, magnified 180 diam.)

The last pair of feet on the middle-body of the larva, consequently the penultimate pair in the adult animal, is almost always similar in structure to the preceding pair. A remarkable exception is, however, presented in this respect by Cryptoniscus and Entoniscus,--remarkable as a confirmation of Darwin's proposition that "parts developed in an unusual manner are very variable," for in the peculiarly-formed pair of feet there exists the greatest possible difference between the three species. .h.i.therto observed. In Cryptoniscus (Figure 39) this last foot is thin and rod-like; in Entoniscus Cancrorum remarkably long and furnished with a strongly thickened hand and a peculiarly constructed chela; in Entoniscus Porcellanae very short, imperfectly jointed, and with a large ovate terminal joint (Figure 40).

Some Isopods undergo a considerable change immediately before the attainment of s.e.xual maturity. This is the case with the males of Tanais which have already been noticed, and, according to Hesse, with the Pranizae, in which both s.e.xes are said to pa.s.s into the form known as Anceus. But Spence Bate, a careful observer, states that he has seen females of the form of Praniza laden with eggs far advanced in their development.

(FIGURE 41. Entoniscus Cancrorum, female, magnified 3 times.

FIGURE 42. Cryptoniscus planarioides, female, magnified 3 times.

FIGURE 43. Embryo of a Corophium, magnified 90 diam.)

In this order we meet for the first time with an extensive retrograde metamorphosis as a consequence of a parasitic mode of life. Even in some Fish-lice (Cymothoa) the young are lively swimmers, and the adults stiff, stupid, heavy fellows, whose short clinging feet are capable of but little movement. In the Bopyridae (Bopyrus, Phryxus, Kepone, etc., which might have been conveniently left in a single genus), which are parasitic on Crabs, Lobsters, etc., taking up their abode chiefly in the branchial cavity, the adult females are usually quite dest.i.tute of eyes; the antennae are rudimentary; the broad body is frequently unsymmetrically developed in consequence of the confined s.p.a.ce; its segments are more or less amalgamated with each other; the feet are stunted, and the appendages of the abdomen transformed from natatory feet with long setae into foliaceous or tongue-shaped and sometimes ramified branchiae. In the dwarfish males the eyes, antennae, and feet, are usually better preserved than in the females; but on the other hand all the appendages of the abdomen have not unfrequently disappeared, and sometimes every trace of segmentation. In the females of Entoniscus, which are found in the body-cavity of Crabs and Porcellanae, the eyes, antennae, and buccal organs, the segmentation of the vermiform body, and in one species (Figure 41) the whole of the limbs, disappear almost without leaving a trace; and Cryptoniscus planarioides would almost be regarded as a Flatworm rather than an Isopod, if its eggs and young did not betray its Crustacean nature. Among the males of these various Bopyridae, that of Entoniscus Porcellanae occupies the lowest place; it is confined all its life to six pairs of feet, which are reduced to shapeless rounded lumps.

The Amphipoda are distinguishable from the Isopoda at an early period in the egg by the different position of the embryo, the hinder extremity of which is bent downwards. In all the animals of this order which have been examined for it,* (* In the genera Orchestoidea, Orchestia, Allorchestes, Montagua, Batea n.g., Amphilochus, Atylus, Microdeutopus, Leucothoe, Melita, Gammarus (according to Meissner and La Valette), Amphithoe, Cerapus, Cyrtophium, Corophium, Dulichia, Protella and Caprella.) a peculiar structure makes its appearance very early on the anterior part of the back, by which the embryo is attached to the "inner egg-membrane," and which has been called the "micropylar apparatus," but improperly as it seems to me.* (* Little as a name may actually affect the facts, we ought certainly to confine the name "micropyle" to ca.n.a.ls of the egg-membrane, which serve for the entrance of the s.e.m.e.n. But the outer egg-membrane pa.s.ses over the "micropylar apparatus" of the Amphipoda without any perforation, according to Meissner's and La Valette's own statements; it appears never to be present before fecundation, attains its greatest development at a subsequent period of the ovular life, and the delicate ca.n.a.ls which penetrate it do not even seem to be always present, indeed it seems to belong to the embryo rather than to the egg-membrane. I have never been able to convince myself that the so-called "inner egg-membrane" is really of this nature, and not perhaps the earliest larva skin, not formed until after impregnation, as might be supposed with reference to Ligia, Ca.s.sidina and Philoscia.) It will remind us of the union of the young Isopoda with the larval membrane and of the unpaired "adherent organ" on the nape of the Cladocera, which is remarkably developed in Evadne and persists throughout life; but in Daphnia pulex, according to Leydig, although present in the young animals, disappears without leaving a trace in the adults.

The young animal, whilst still in the egg, acquires the full number of its segments and limbs. In cases where segments are amalgamated together, such as the last two segments of the thorax in Dulichia, the last abdominal segments and the tail in Gammarus ambulans and Corophium dentatum, n. sp., and the last abdominal segments and the tail in Brachyscelus,* or where one or more segments are deficient, as in Dulichia and the Caprellae, we find the same fusion and the same deficiencies in young animals taken out of the brood-pouch of their mother. (* According to Spence Bate, in Brachyscelus crusculum the fifth abdominal segment is not amalgamated with the sixth (the tail) but with the fourth, which I should be inclined to doubt, considering the close agreement which this species otherwise shows with the two species that I have investigated.) Even peculiarities in the structure of the limbs, so far as they are common to both s.e.xes, are usually well-marked in the newly hatched young, so that the latter generally differ from their parents only by their stouter form, the smaller number of the antennal joints and olfactory filaments, and also of the setae and teeth with which the body or feet are armed, and perhaps by the comparatively larger size of the secondary flagellum. An exception to this rule is presented by the Hyperinae which usually live upon Acalephae. In these the young and adults often have a remarkably different appearance; but even in these there is no new formation of body-segments and limbs, but only a gradual transformation of these parts.*

(* In the young of Hyperia galba Spence Bate did not find any of the abdominal feet, or the last two pairs of thoracic feet, but this very remarkable statement required confirmation the more because he examined these minute animals only in the dried state. Subsequently I had the wished-for opportunity of tracing the development of a Hyperia which is not uncommon upon Ctenophora, especially Beroe gilva, Eschsch. The youngest larva from the brood-pouch of the mother already possess THE WHOLE of the thoracic feet; on the other hand, like Spence Bate, I cannot find those of the abdomen. At first simple enough, all these feet soon become converted, like the anterior feet, into richly denticulated prehensile feet, and indeed of three different forms, the anterior feet (Figure 44) the two following pairs (Figure 45) and finally the three last pairs (Figure 46) being similarly constructed and different from the rest. In this form the feet remain for a very long time, whilst the abdominal appendages grow into powerful natatory organs, and the eyes, which at first seemed to me to be wanting, into large hemispheres. In the transition to the form of the adult animal the last three pairs of feet (Figure 49) especially undergo a considerable change. The difference between the two s.e.xes is considerable; the females are distinguished by a very broad thorax, and the males (Lestrigonus) by very long antennae, of which the anterior bear an unusual abundance of olfactory filaments.

Their youngest larvae of course cannot swim; they are helpless little animals which firmly cling especially to the swimming laminae of their host; the adult Hyperiae, which are not unfrequently met with free in the sea, are, as is well known, the most admirable swimmers in their order. ("Il nage avec une rapidite extreme," says Van Beneden of H.

Latreillii M.-Edw.)

The transformation of the Hyperiae is evidently to be regarded as ACQUIRED and not INHERITED, that is to say the late appearance of the abdominal appendages and the peculiar structure of the feet in the young are not to be brought into unison with the historical development of the Amphipoda, but to be placed to the account of the parasitic mode of life of the young.

As in Brachyscelus, free locomotion has been continued to the adult and not to the young, contrary to the usual method among parasites. Still more remarkable is a similar circ.u.mstance in Caligus, among the parasitic Copepoda. The young animal, described by Burmeister as a peculiar genus, Chalimus, lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When s.e.xual maturity is attained, the cable is cut, and the adult Caligi, which are admirable swimmers, are not unfrequently captured swimming freely in the sea. (See 'Archiv. fur Naturgeschichte' 1852 1 page 91).)

(FIGURES 44 TO 46. Feet of a half-grown Hyperia Martinezii, n. sp.

(Named after my valued friend the amiable Spanish zoologist, M.

Francisco de Paula Martinez y Saes, at present on a voyage round the world.)

FIGURES 47 TO 49. Feet of a nearly adult male of the same species; 44 and 47 from the first pair of anterior feet (gnathopoda); 44 and 48 from the first, and 46 and 49 from the last pair of thoracic feet. Magnified 90 diam.)

Thus, in order to give a few examples, the powerful chelae of the antepenultimate pair of feet, of Phromina sedentaria, are produced, according to Pagenstecher, from simple feet of ordinary structure; and vice versa, the chelae on the penultimate pair of feet of the young Brachyscelus, become converted into simple feet. In the young of the last-mentioned genus the long head is drawn out into a conical point and bears remarkably small eyes; in course of growth, the latter, as in most of the Hyperinae, attain an enormous size, and almost entirely occupy the head, which then appears spherical, etc.

The difference of the s.e.xes which, in the Gammarinae is usually expressed chiefly in the structure of the anterior feet (gnathopoda, Sp.

Bate) and in the Hyperinae in the structure of the antennae, is often so great that males and females have been described as distinct species, and even repeatedly placed in different genera (Orchestia and Talitrus, Cerapus and Dercothoe, Lestrigonus and Hyperia) or even families (Hyperines anormales and Hyperines ordinaires). Nevertheless it is only developed when the animals are nearly full-grown. Up to this period the young resemble the females in a general way, even in some cases in which these differ more widely than the males from the "Type" of the order.

Thus in the male Sh.o.r.e-hoppers (Orchestia) the second pair of the anterior feet is provided with a powerful hand, as in the majority of the Amphipoda, but very differently constructed in the females. The young, nevertheless, resemble the female. Thus also,--and this is an extremely rare case,* (* "I know of no case in which the inferior (antennae) are obsolete, when the superior are developed," Dana.

(Darwin, 'Monograph on the Subcla.s.s Cirripedia, Lepadidae' page 15.)--the females of Brachyscelus are dest.i.tute of the posterior (or inferior) antennae; the male possesses them like other Amphipodae; in the young I, like Spence Bate, can find no trace of them.

It is, however, to be particularly remarked, that the development of the s.e.xual peculiarities does not stand still on the attainment of s.e.xual maturity.

(FIGURE 50. Foot of the second pair ("second pair of gnathopoda") of the male of Orchestia Tucurauna, magnified 15 diam.