Jaekel refused to believe that the antennae of trilobites were really entirely simple, and so h.o.m.ologized them with the antennae and not the antennules of other Crustacea. In this he was obviously incorrect, but it accounts for his h.o.m.ology of the remainder of the cephalic appendages.

It is, at present, impossible to demonstrate the actual number of somites in the cephalon of the trilobite, but I believe that Beecher was correct in holding that the glabella was composed of four segments. There are, it is true, a number of trilobites (Mesonacidae, Paradoxidae Cheiruridae, etc.) which show distinctly four pairs of glabellar furrows, indicating five segments in the glabella. This is, however, probably due to a secondary division of the first lobe.

The correspondence of the five segments on the dorsal side with the five pairs of appendages makes it unlikely that any pair of limbs has been lost. The condition in _Marrella_, where a trilobite-like cephalon bears five pairs of appendages, the second pair of which are tactile antennae, is favorable to the above interpretation. In spite of the apparent degeneration of the first two pairs of appendages in _Calymene_, no limbs are actually missing, and if some are dropped out in the later trilobites it would not affect the h.o.m.ology of those now known. I therefore agree with Beecher in h.o.m.ologizing the appendages, pair for pair, with those of the higher Crustacea.

FUNCTIONS OF THE APPENDAGES.

_Antennules._

The antennules were obviously tactile organs, probably freely movable in most trilobites, but in the case of Triarthrus perhaps rather rigid, judging from the great numbers of specimens which show the characteristic sigmoid curve made familiar by Professor Beecher's restoration. The proximal end of the shaft of each antennule of Triarthrus is hemispheric and doubtless fitted into a socket, thus suggesting great mobility of the whole organ. In spite of this, I have seen no specimens in which they did not turn in toward each other and cross the anterior margin very near the median line. In front of the margin, various specimens show evidence of flexibility, but from the proximal end to the margin the position is the same in all specimens.

In all the few specimens of _Cryptolithus_ retaining the antennules, these organs are turned directly backward, but it is entirely within the range of probabilities that while its burrowing habits made this the more usual position, the animal had the power of turning them around to the front when they could be used to advantage in that direction.

_Exopodites._

It has been the opinion of most observers that the exopodites of trilobites were swimming organs, while others have thought that they functioned also in aerating the blood. To the present writer it seems probable that the chief function was that of acting as gills, for which the numerous thin, flattened or blade-like setae are particularly adapted. That they were also used in swimming is of course possible, but that was not their chief function. It should be remembered that the exopodites are always found dorsal to or above the endopodites, and in a horizontal plane. For use in swimming it would have been necessary to rotate each exopodite into a plane approximately perpendicular to or at least making a considerable angle with the dorsal test. In this position, the exopodites would have been thrust down between the endopodites, and one would expect to find some specimens in which a part at least of the exopodites were ventral to the endopodites. Specimens in this condition have not yet been seen among the fossils. To avoid having the exopodites and endopodites intermingled in this way, the animal would have to bring all the endopodites together along the axial line in a plane approximately perpendicular to the dorsal test, in which case the exopodites would be free to act as swimming organs. The fact that the setae of an exopodite stay together like the barbs on a feather would of course tend to strengthen the idea that the exopodites could be used in swimming, but that is not the only possible explanation of this condition. The union of the basipodite and exopodite shows that the two branches of the appendage acted together. Every movement of one affected the other, and the motion of the endopodites in either swimming or crawling produced a movement of the exopodites which helped to keep up a circulation of water, thus insuring a constant supply of oxygen.

Although _Neolenus_ is usually accounted a less primitive form than _Ptychoparia_ or _Triarthrus_, it has much the most primitive type of exopodite yet known. It would appear that the exopodites were originally broad, thin, simple lamellae, which became broken up, on the posterior side, into fine cylindrical setae. As development progressed, more and more of the original lamella was broken up until there remained only the anterior margin, which became thickened and strengthened to support the delicate filaments. The setae in turn became modified from their original simple cylindrical shape to form the wide, thin, blade-like filaments of _Cryptolithus_ and _Ceraurus_.

Another possible use of the exopodites is suggested by the action of some of the barnacles, which use similar organs as nets in gathering food and the endopodites as rakes which take off the particles and convey them to the mouth. The exopodites of the trilobite might well set up currents which would direct food into the median groove, where it could be carried forward to the mouth.

_Endopodites._

The endopodites were undoubtedly used for crawling; in some trilobites, probably most of them, for swimming; in the case of _Cryptolithus_, and probably others, for burrowing; and probably in all for gathering food, in which function the numerous spines with which they are arrayed doubtless a.s.sisted.

Various trails have been ascribed to the action of trilobites, and many of them doubtless were made by those animals (see especially Walcott, 1918). Some of these trails seem to indicate that in crawling the animal rested on the greater part of each endopodite, while others, notably the _Protichnites_ recently interpreted by Walcott (1912 B, p. 275, pl. 47), seem to have touched only the spinous tips of the dactylopodites to the substratum. The question of the tracks, trails, and burrows which have been ascribed to trilobites is discussed briefly on a later page; but can not be taken up fully, as it would require another monograph to treat of them satisfactorily.

The flattened, more or less triangular segments of the endopodites of the posterior part of the thorax and pygidium in _Triarthrus_, _Cryptolithus_, and _Acidaspis_ probably show an adaptation of the endopodites of the posterior part of the body both as more efficient pushing organs and as better swimming legs. The fact that these segments are pointed below enabled them to get a better grip on whatever they were crawling over, and the flattening allowed a much greater surface to be opposed to the water in swimming. In this connection it might be stated that it seems very probable that the trilobites with large pygidia at least, perhaps all trilobites, had longitudinal muscles which allowed them to swim by an up and down motion of the fin-like posterior shield, the pygidium acting like the caudal fin of a squid. Such a use would explain the function of the large, nearly flat pygidia seen in so many of the trilobites beginning with the Middle Cambrian, and of those with wide concave borders. It should be noted here that it is in trilobites like _Isotelus_, with pygidia particularly adapted to this method of swimming, that the endopodites are most feebly developed, and show no flattening or modification as swimming organs.

The relatively strong, curved, bristle-studded endopodites of _Cryptolithus_, combined with its shovel-shaped cephalon, indicate _Limulus_-like burrowing habits for the animal, and the mud-filled casts of its intestine corroborate this view. That it was not, however, entirely a mud groveller is indicated by its widespread distribution in middle Ordovician times.

_Use of the Pygidium in Swimming._

The idea that the use of the pygidium as a swimming organ is a possible explanation of that caudalization which is so characteristic of trilobites has not been developed so far as its merits seem to deserve. Two princ.i.p.al uses for a large pygidium of course occur to one: either it might form a sort of operculum to complete the protection when the trilobite was enrolled, or it might serve as a swimming organ. That the former was one of its important functions is shown by the nicety with which the cephalon and pygidium are adapted to one another in such families as the Agnostidae, Asaphidae, Phacopidae, and others. That a large pygidium is not essential to perfect protection on enrollment is shown by an equally perfect adjustment of the two shields in some families with small pygidia, notably the Harpedidae and Cheiruridae That the large pygidial shields are not for protective purposes only is also shown by those forms with large pygidia which are not adjusted to the conformation of the cephalon, as in the Goldiidae and Lichadidae. It is evident that a large pygidium, while useful to complete protection on enrollment, is not essential.

It would probably be impossible to demonstrate that the trilobites used the pygidium in swimming. The following facts may, however, be brought forward as indicating that they probably did so use them.

1. The appendages, both exopodites and endopodites, are relatively feebly developed as swimming organs. This has been discussed above, and need not be repeated. It must in fairness be observed, however, that many modern Crustacea get about very well with limbs no better adapted for swimming than those of the trilobites.

2. The articulations of the thoracic segments with each other and with the two shields are such as to allow the pygidium to swing through an arc of at least 270, that is, from a position above the body and at right angles to it, around to the plane of the bottom of the cephalon.

Specimens are occasionally found in which the thorax and pygidium are so flexed that the latter shield stands straight above the body. A well preserved _Dipleura_ in this position is on exhibition in the Museum of Comparative Zoology, and Mr. Narraway and I have figured a _b.u.mastus milleri_ in the same att.i.tude (Ann. Carnegie Mus., vol. 4, 1908, pl. 62, fig. 3).

3. What little can be learned of the musculature (see under musculature, seq.) indicates that the trilobites had powerful extensor and flexor muscles, such as would be required for this method of swimming. It may be objected that the longitudinal muscles were too small to permit the use of a caudal fin. In the lobster, where this method of progression is most highly developed, there is a large ma.s.s of muscular tissue which nearly fills the posterior segments.

Trilobites have not usually been thought of as powerfully muscled, but it may be noted that in many cases broad axial lobes accompany large pygidia. As the chief digestive region appears to have been at the anterior end, and other organs are not largely developed, it seems probable that the great enlargement of the axial lobe was to accommodate the increased muscles necessary to properly operate the pygidium. It may be noted that in all these genera the axial lobe of the pygidium is either short or narrow.

4. The geological history of the rise of caudalization favors this view. With the exception of the Agnostidae and Eodiscidae, all Lower Cambrian trilobites had small pygidia, and the same is true of those of the Middle Cambrian of the Atlantic realm (except for the _Dorypyge_ of Bornholm). In Pacific seas, however, large-tailed trilobites of the families Oryctocephalidae, Bathyuridae, and Asaphidae then began to be fairly common, though making up but a small part of the total fauna of trilobites. In the Upper Cambrian of the Atlantic province the Agnostidae were the sole representatives of the isopygous trilobites, while in the Pacific still another family, the Dikelocephalidae, was added to those previously existing.

With the Ordovician, caudalization reached its climax and the fashion swept all over the world. It is shown not so much in the proportion of families with large pygidia, as in the very great development of the particular trilobites so equipped. Asaphidae and Illaenidae were then dominant, and the Proetidae, Cyclopygidae Goldiidae, and Lichadidae had begun their existence. A similar story is told by the Silurian record, except that the burden of the Asaphidae has been transferred to the Lichadidae and Goldiidae. All the really old (Cambrian) families of trilobites with small pygidia had now disappeared. In the general dwindling of the subcla.s.s through the Devonian and later Palaeozoic, the few surviving species with small pygidia were the first to go, and the proetids with large abdominal shields the last.

The explanation of this history is probably to be found in the rise of the predatory cephalopods and fishes, the natural enemies of the trilobites, against whom they could have no other protection than their agility in escaping. While the records at present known carry the fishes back only so far as the Ordovician (fishes may have arisen in fresh waters and have gone to sea in a limited way in the Ordovician and more so in Silurian time) and the cephalopods to the Upper Cambrian, the rise of the latter must have begun at an earlier date, and it is probably no more than fair to conjecture that the attempt to escape swimming enemies caused an increase in the swimming powers of the trilobites themselves. At any rate, the time of the great development of the straight cephalopods coincided with the time of greatest development of caudalization; both were initiated in the Pacific realm, and both spread throughout the marine world during the middle Ordovician. And since, in the asaphids, a decrease in swimming power of the appendages accompanied the increase in the size of the pygidium, it seems probable that the swimming function of the one had been transferred to the other. A high-speed, erratic motion which could be produced by the sudden flap of a pygidium would be of more service in escape than any amount of steady swiftness produced by the oar-like appendages of an animal of the shape of a trilobite.

_c.o.xopodites._

The primary function of the endobases of the c.o.xopodites was doubtless the gathering, preparation, and carrying of food to the mouth.

Although the endobases of opposite sides could not in all cases meet one another, they were probably spinose or setiferous and could readily pa.s.s food from any part of the axial groove forward to the mouth, and also send it in currents of water. The endobases of the cephalic c.o.xopodites were probably modified as gnathites in all cases, but little is known of them except in Triarthrus, where they were flattened and worked over one another so as to make excellent shears for slicing up food, either animal or vegetable. In some cases the proximal ends of opposed gnathites were toothed so as to act as jaws, but a great deal still remains to be learned about the oral organs of all species.

The writer has suggested (1910, p. 131) that a secondary function of the endobases of the thorax of _Isotelus_ and probably other trilobites with wide axial lobes was that of locomotion. In _Isotelus_ the endobases of the thorax are greatly over-developed, each being much stouter and nearly as long as the corresponding endopodite, and the explanation seemed to me to lie in the locomotor or crawling use of these organs instead of the endopodites. Certain trails which I figured seemed to support this view.

POSITION OF THE APPENDAGES IN LIFE.

In almost all the specimens so far recovered the appendages are either flattened by pressure or lie with their flat surfaces in or very near the plane of stratification of the sediment. This flattening is extreme in Neolenus, Ptychoparia, and Kootenia, moderate in _Triarthrus_ and _Cryptolithus_, and apparently slight or not effective in _Isotelus_, _Ceraurus_, and _Calymene_. These last are, however, from the conditions of preservation, least available for study.

In Part IV, attention is called to a specimen of Triarthrus (No. 222) in which some of the endopodites are imbedded nearly at right angles to the stratification of the shale. This specimen is especially valuable because it shows that the appendages in the average specimen of Triarthrus have suffered very little compression, and it also suggests the probable position of the endopodites when used for crawling.

In considering the position of the appendages in life, one must always remember one great outstanding feature of trilobites, the thinness and flexibility of the ventral membrane. The appendages were not inserted in any rigid test but were held only by muscular and connective tissue. Hence we must premise for them great freedom of motion, and also relatively little power. The rigid appendifers, and the supporting apodemes discovered by Beecher, supplied fulcra against which they could push, but their attachment to these was rather loose.

Considering, first, the position of the appendages in crawling, it appears that different trilobites used their appendages in different ways. _Neolenus_ had compact stocky legs, which allowed little play of one segment on another, as is shown by the wide joints at right angles to the axis of the segment. Such limbs were stiff enough to support the body when the animal was crawling beneath the water, where of course it weighed but little. That such a crawling att.i.tude was adopted by trilobites has been shown by Walcott in his explanation of the trails known as _Protichnites_ (1912 B, p. 278). Many trilobites probably crawled in this way, on the tips of the toes, so to speak.

In such the limbs would probably extend downward and outward, with the flattened sides vertical.

The limb of _Triarthrus_, however, is of another type. The endopodites are long, slender, flexibly jointed, the whole endopodite probably too flexible to be used as a unit as a leg must be in walking on the "toes." The proximal segments of the thoracic and pygidial endopodites are, however, triangular instead of straight-sided, and, the spine-bearing apex of the triangle being ventral, it enabled the endopodites to get a grip on the bottom and thus push the animal forward. This method of progression was more clumsy and less rapid than that of Neolenus, but it sufficed. The natural position of the endopodite when used in this way would seem to be with the flattened sides of the segments standing at an angle of 30 to 45 with the vertical, thus allowing a good purchase on the bottom and at the same time offering the minimum resistance to the water when moving the appendages forward.

_Isotelus_ has endopodites different from those of either _Neolenus_ or _Triarthrus_. They are composed of cylindrical segments, the joints indicating a certain amount of flexibility. Since there is no method by which the segments may get a purchase on the bottom other than by pushing with the distal ends, it would seem at first thought that _Isotelus_, like Neolenus, crawled on its "toes." The endopodites of _Isotelus_ are however, short and feeble when compared with the size of the test, while the endobases of the c.o.xopodites are extraordinarily developed. These facts, together with certain trails, strongly suggest the use of the c.o.xopodites as the primary ambulatory organs, the endopodites probably a.s.sisting. In this event, the position of the endopodites and c.o.xopodites would be downward, both outward and inward from the point of attachment, and the motion both backward and forward. The fact that in the specimens as preserved the c.o.xopodites point backward and the endopodites forward indicates that the limb as a whole swung on a pivot at the appendifer. It is of course natural to suggest that the c.o.xopodites and endopodites of all the trilobites with wide axial lobes, _Nileus_, _b.u.mastus_, _Homalonotus_, etc., were developed in this same way.

_Cryptolithus_ presents still another and very peculiar development of the endopodites where ability to get purchase on the sea floor is obtained by a stout limb of slight flexibility, bowed and turned backward in the middle, where an enlarged segment insures stiffness.

The segments are flattened, and since the greatest strength when used in pushing and crawling is in the long axis of the oval section of the flattened limb, it seems probable that these limbs did not hang directly down, with their sides vertical, but that their position in life was very much the same as that in which they are preserved as fossils. By moving these bowed legs forward and backward in a plane at a small angle to the surface of the body, a powerful pushing impetus could be obtained. They may, however, have occupied much the same position as do those of _Limulus_.

In the case of the endopodites, therefore, it is necessary to study the structure and probable method of their use in each individual genus before suggesting what was the probable position in life. In the act of swimming, the position was probably more uniform. When the endopodites were used in swimming, as they undoubtedly could be with more or less effect in all the trilobites now known, those with flattened surfaces probably had them at such an angle as to give the best push against the water on the back stroke, while on the forward stroke the appendage would be turned so that' the thin edge opposed the water. The great flexibility of attachment would certainly permit this, though unfortunately nothing is as yet known of the musculature. The c.o.xopodites of course had less freedom of movement in this respect, and probably could not turn their faces. For this reason, it seems to me likely that those c.o.xopodites which are compressed did not stand with their flattened faces vertical, but in a position which was nearly horizontal or at least not more than 45 from the horizontal. If the flattened faces were vertical, they would be in constant opposition to the water during forward movements and would be of no use in setting up currents of water toward the mouth, as every back stroke would reverse the motion.

The position of the exopodites in life seems to have been rather uniform in all the genera now known. I have set forth on a previous page my reasons for thinking that they took little part in swimming, and I look upon them as being, in effect, leaf-gills. It seems probable that in all genera the exopodites were held rather close to the test, the shaft more or less rigid, the filamentous setae gracefully pendent, but pendent as a sheet and not individually, there having been some method by which adjoining setae were connected laterally. Free contact with the water was thus obtained without the mingling of endopodites and exopodites which would have been so disastrous to progression.

PART II.

Structure And Habits Of Trilobites.

INTERNAL ORGANS AND MUSCLES.

Granting that the trilobite is a simple, generalized, ancient crustacean, it appears justifiable to attribute to it such internal organs as seem, from a study of comparative anatomy, to be primitive.

The alimentary ca.n.a.l would be expected to be straight and simple, curving downward to the mouth, and should be composed of three portions, stomodaeum, mesenteron, and proctodaeum, the first and last with chitinous lining. In modern Crustacea, muscle-bands run from the gut to part of the adjacent body wall, so that scars of attachment of these muscles may be sought. At the anterior end of the stomodaeum, they are usually especially strong. From the mesenteron there might be pouch-like or tubular outgrowths.