Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pl. 11;—31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl. 1, fig. 3;—Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl. 4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol. 67,1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12 (notCalymene), (not 15,Calymene); pl. 28, figs. 1-5; pl. 34, fig. 1; pl. 35, fig. 7.—Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.—Bernard, The Apodidæ, 1892, text figs. 46, 51.
Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pl. 11;—31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl. 1, fig. 3;—Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl. 4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol. 67,1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12 (notCalymene), (not 15,Calymene); pl. 28, figs. 1-5; pl. 34, fig. 1; pl. 35, fig. 7.—Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.—Bernard, The Apodidæ, 1892, text figs. 46, 51.
Cephalic Appendages.
No trace of antennules has yet been found.
I find only three sections cut through the plane of the hypostoma of Ceraurus which show anything of the cephalic appendages, and no one of them is very satisfactory. The best is No. 22, the one figured by Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should remember that this section is not actually cut in the plane of the hypostoma but is a slice diagonally through the head, cutting through one eye and the posterior end of the hypostoma. It shows what seem to be the coxopodites of the second, third, and fourth pairs of cephalic appendages, the exopodites of the third and fourth pairs, and the metastoma. If this interpretation is correct, the first pair of gnathites lay alongside the hypostoma or under its edge, and were feebly developed, the second pair were attached in front of the tip of the hypostoma, curved back close to it, and their inner ends reached the sides of the metastoma. The third and fourth pairs were back of the metastoma, the third pair was stronger than the second, and the fourth probably like the third.
Fig. 17.Transverse section ofCeraurus pleurexanthemus, showing the relation of the coxopodite to the appendifer. Traced from a photographic enlargement of the slice. Specimen 128. × 4/5.
Fig. 17.Transverse section ofCeraurus pleurexanthemus, showing the relation of the coxopodite to the appendifer. Traced from a photographic enlargement of the slice. Specimen 128. × 4/5.
Fig. 18.Slice ofCeraurus pleurexanthemus, showing a nearly continuous section of an endopodite and an exopodite above it. The latter is so cut as to show only the edge of the shaft and the bases of a few setæ. Traced from a photographic enlargement. Specimen in. × 4.
Fig. 18.Slice ofCeraurus pleurexanthemus, showing a nearly continuous section of an endopodite and an exopodite above it. The latter is so cut as to show only the edge of the shaft and the bases of a few setæ. Traced from a photographic enlargement. Specimen in. × 4.
Specimen 92 shows traces of the slender endopodites belonging to the cephalon, but no details. Specimen 22 shows on one side exopodites (epipodites of Walcott) belonging to the third and fourth cephalic appendages. That belonging to the third shows some long setæ and a trace of the shaft, while the one on the fourth appendage (third coxopodite) has a portion of a broad shaft and a number of long setæ. It should again be remembered that the slice does not cut through the plane of the exopodite, but across it at a low angle, so that a part but not all of the shaft is shown. On the other side of this slice there is a fairly good section of one of the thoracic exopodites. It is, however, turned around in the opposite direction from the others, as would be expected in an enrolled specimen.
Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally through the head of Ceraurus, in front of the posterior tip of the hypostoma. They show fragments of endopodites and exopodites which may be interpreted as practically identical in form with those of the thorax. Due to the diagonal plane in which the section is cut, slice 5 showsthe coxopodites of two pairs of appendages, one lying nearer the median cavity than the other. It is extremely difficult to visualize the interpretation of such sections.
Thoracic Appendages.
A transverse section through a thoracic segment (No. 128, our fig. 17) shows the relation of coxopodite to appendifer to be the same as inCalymene, the upper side of the coxopodite having a notch a little outward from the middle. After seeing that specimen, it is possible to understand slice No. 168, which shows longitudinal sections through a number of coxopodites of the thorax, with fragments of both exopodites and endopodites articulated at the distal ends. These and longitudinal vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the endobases taper inward, and the general uniformity in width in sections taken at various angles indicates that the coxopodites were not greatly flattened.
A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918; our fig. 18) shows a nearly complete thoracic endopodite, and above it a part of the proximal end of the exopodite of the same segment. When one considers that out of over two thousand sections only this one shows the six successive segments of an endopodite, one realizes how futile it is to expect that dozens of the equally slender "spirals" should be cut so as to show practically all their turns.
This endopodite is slender, all the segments have nearly the same length and diameter, though there is a slight taper outward, each segment is expanded distally for the articulation of the next, and there are small spines on the distal ends of some of them. There is probably a terminal spine present, though it is neither so long nor so plainly visible as in Walcott's photograph.
The exopodite on this same specimen was evidently cut diagonally across near the setiferous edge, showing a section through the shaft and the bases of seven setæ (fig. 18). This section is so exactly what would be obtained by cutting similarly an exopodite of either Neolenus orTriarthrusthat it should in itself dispose of the "spiral-exopodite" theory.
Several sections have already been illustrated showing sections across the setæ of the exopodites (pl. 3, figs. 4-6, 1881; pl. 27, figs. 3, 4, 9, 1918), and similar sections are not uncommon. Only a very few, however, show sections in the plane of the exopodite. If only No. 111, described above, were known, it would be inferred that the exopodite had a slender shaft as inCalymene, but another good slice, No. 80 (fig. 12, ante) shows that the blade was rather broad, though not so broad as in Neolenus. The other specimen is No. 22, which has already been discussed. The thoracic exopodite of this specimen has been very incorrectly figured by Walcott, as it shows no such palmate shaft as he has indicated, but a long blade-like one is outlined, though its entire width is not actually shown.
Pygidial Appendages.
Sections 14 and 18 (pl. 2, figs. 4, 8, 1881) prove the presence under the pygidium of three pairs of appendages, the coxopodites and fragments of endopodites of which are shown. Nothing is known of the exopodites.
Relation of Hypostoma to Cephalon.
In Ceraurus the body portion and posterior end of the hypostoma are roughly oval, about as wide as the glabella at its broadest part, and the posterior edge extends back towithin 0.5 to 1 mm. of the neck furrow. The posterior pair of appendifers are behind the hypostoma, while the second pair are in front of its posterior end but escape being covered by it on account of its oval shape. At the anterior end the hypostoma is widened by the presence of two side lappets which extend beyond the boundaries of the glabella. In both Ceraurus and Cheirurus the anterior edge of the hypostoma fits against the doublure at the anterior margin of the head and the epistoma is either entirely absent or is so narrow as not to be seen in specimens in the ordinary state of preservation. A section across the cephalon ofCeraurus pleurexanthemusat the horizon of the eyes shows the sides of the hypostoma fitting closely against the sides of the glabella (Walcott's pl. 1, fig. 1). Further back on the head it is not in contact with the dorsal test, and the gnathobases extend beneath it.
Restoration of Ceraurus pleurexanthemus.
(pl. 11; textfig. 19.)
The restoration of the appendages ofCeraurus pleurexanthemusis a tentative one, based upon a careful study of the translucent sections prepared by Doctor Walcott. In no case among these sections is the actual test of any appendage preserved, and the real form of each part is generally obscured by the crystallization of the calcite which fills the spaces formerly occupied by animal matter.
Fig. 19.Restoration of a transverse section of the thorax ofCeraurus pleurexanthemusGreen, showing the relation of the appendages to the appendifers and the ventral membrane. The probable positions of the heart and alimentary canal are indicated.
Fig. 19.Restoration of a transverse section of the thorax ofCeraurus pleurexanthemusGreen, showing the relation of the appendages to the appendifers and the ventral membrane. The probable positions of the heart and alimentary canal are indicated.
No section shows anything which can be identified as any part of the antennules, so that these organs have been supplied from analogy withTriarthrus.
There are undoubtedly four pairs of biramous Cephalic appendages, but their points of attachment are not so obvious. There are two pairs of conspicuous appendifers on the posterior part of the cephalon and another pair almost concealed by the hypostoma. It is probable that the appendages of the cephalon were not attached directly beneath them, as the four pairs have to be placed within the space occupied by the three pairs of appendifers. As the mouth is in front of the posterior end of the hypostoma, the gnathites of the first pair of biramous appendages may have extended beneath that organ, or they may have lain beside it, and only become functional when the hypostoma was dropped down in the feeding position. The second pair of gnathites reached just to the tip of the hypostoma, and the other two pairs seemingly curved backward behind it.
The points of attachment on the thorax, as shown clearly in sections, were directly beneath the lower ends of the appendifers. The endopodites were long enough to reach to or a little beyond the outer extremities of the pleural spines, while the exopodites were apparently somewhat shorter. Each endopodite consisted of six short, fairly stout segments, each with at least two spines on the somewhat expanded distal ends. The exactform of the exopodites could not be made out. The shaft was apparently rather short, unsegmented, and fairly broad. The setæ appear from the sections to have been more or less blade-shaped and to have overlapped, as do those of the exopodites ofCryptolithus. Judging from their position in the sections, the setæ not only bordered the posterior side of the shaft, but radiated out from the end as well.
The pygidium shows three pairs of functional appendifers, hence three pairs of appendages have been supplied. There is a fourth pair of rudimentary appendifers, but as they are beneath the doublure they could not have borne ambulatory appendages.
The Appendages of Acidaspis trentonensis Walcott.
(pl. 6, fig. 6.)
A single individual ofAcidaspis trentonensis, obtained from the same locality and horizon as the specimens ofTriarthrusandCryptolithus, when cleaned from the ventral side shows a number of poorly preserved endopodites which seem very similar in shape and position to those ofTriarthrus. One endopodite on the right side of the head and the first five on the right side of the thorax are the best shown. All are slender, are directed first forward at an angle of about 45 with the axis, then, except in the case of the cephalic appendage, turn backward on a gentle curve and extend a little distance beyond the margin of the test, but not as far as the tips of the lateral spines of the thoracic segments.
The individual segments of the endopodites can not be seen clearly enough to make any measurements. On the fourth and fifth endopodites of the thorax, some of the segments seem to be broad and triangular as inTriarthrus. All that can be seen indicates thatAcidaspishad appendages entirely similar to those ofTriarthrus, but perhaps not quite so long, as they seem not to have projected beyond the limits of the lateral spines. There are no traces of antennules nor, unfortunately, of exopodites.
Measurements:Length 8 mm.
Walcott (1881, p. 206) stated that his sections had shown the presence in this species of legs "both cephalic and thoracic" and also the "spiral branchiæ." His specimens were from the Trenton at Trenton Falls, New York.
Cryptolithus tessellatusGreen.
(pl. 6, fig. 7; pls.7-9; text figs.20,25,45,46.)
(See alsoPart IV.)
Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.
When Professor Beecher wrote his short article on the "Structure and Appendages ofTrinucleus" (1895), he had only three specimens showing appendages. In his later work he cleaned several more, so that there are now thirteen specimens ofTrinucleus=Cryptolithusavailable for study, though some of these do not show much detail. In his last and unpublished study, Beecher devoted the major part of his attention to this genus, and summarized his findings in the drawings which he himself made of the best individuals (text figs.45,46). Valiant (1901) stated that he had found aTrinucleuswith antennæ in the Frankfort shale south of Rome, New York. The specimen has not been figured.
None of the specimens shows much more of the appendages of the cephalon than, the hypostoma and the antennules, so that we are still in ignorance about the mouth parts.
The most striking characteristics of the appendages are as follows: the antennules are long, and turn backward instead of forward; none of the limbs projects beyond the margin of the dorsal test; the exopodites extend beyond the endopodites, reaching very nearly to the margin of the test; the endopodites are not stretched out at right angles to the axis, but the first three segments have a forward and outward direction as inTriarthrus, while the last four turn back abruptly so that they are parallel to the axis; the limbs at the anterior end of the thorax are much more powerful than the others; the dactylopodites of the endopodites show a fringe of setæ instead of three spines as inTriarthrusandNeolenus. All these would, as Beecher has already suggested, seem to be adaptations to a burrowing habit of life, the antennules being turned backward and the other appendages kept within the shelter of the dorsal test in order to protect them, and the anterior endopodites enlarged and equipped with extra spines to make them more efficient digging and pushing organs.
Restoration of Cryptolithus.
(Textfig. 20.)
It should be definitely understood that the present figure is a restoration and not a drawing of a specimen, and that there are many points in the morphology ofCryptolithusabout which no information is available, especially about the appendages under the central portion of the cephalon. The information afforded by all the figures published in this memoir is combined here. As gnathites are preserved on none of the specimens, those represented in the figure are purely conventional.
A person who is acquainted only withCryptolithuspreserved in shale, or with figures, usually has a very erroneous idea of the fringe It is not a flat border spread out around the front of the head, but stands at an angle about 45 in uncrushed specimens of most species. When viewed from the lower side, there is a single outer, concentric row of the cup-shaped depressions, bounded within by a prominent girder. This row is in an approximately horizontal plane, while the remainder of the doublure of the fringe rises steeply into the hollow of the cephalon. Since the front of the hypostoma is attached to this doublure, it stands high up within the vault and under the glabella. Two specimens, Nos. 231 and 233, show something of the hypostoma, and they are the only ones known of any American trinucleid. That of specimen 233, the better preserved, is very small, straight across the front, and oval behind. It seems that it is abnormally small in this specimen and I should not be surprised if in other specimens it should be found to be larger.
In the BohemianTrinucleoides reussi(Barrande), the oldest of the trinucleids, the hypostoma is very commonly present, and is of the proper size to just cover the cavity of the glabella, seen from the lower side, and has, toward the anterior end, side flaps which reach out under the prominent glabellar lobes. This large size of the hypostoma would cause the antennules to be attached outside the dorsal furrows, and the position in which they are attached in the American species ofCryptolithusmay be explained as an inherited one, since with the small hypostoma they might have been within the glabella, as inTriarthrus.
The antennules are seen in three specimens, and in all cases are directed backward. The particular course in which they are drawn in the restoration is purely arbitrary. The second pair of cephalic appendages are represented as directed downward and forward, sincein one or two specimens fragments of forward-pointing endopodites were seen near the front of the cephalon, and because in other trilobites the second pair of appendages is always directed forward. The remaining three pairs have a more solid basis in observed fact, for the two or three specimens retaining fragmentary remains of them indicate that they turn backward like those on the thorax, and that the individual segments are longer and more nearly parallel-sided than those of the more posterior appendages. The gnathites of all the cephalic appendages are admittedly purely hypothetical. None of the specimens shows them. As drawn, they are singularly inefficient as jaws, but if, as is suggested by the casts of the intestines of trinucleids found in Bohemia, these trilobites were mud-feeders, inefficient mouth-parts would be quite in order.
Fig. 20.Cryptolithus tessellatusGreen. A restoration of the appendages drawn by Doctor Elvira Wood from the original specimens and from the photographs made by Professor Beecher. × 9.
Fig. 20.Cryptolithus tessellatusGreen. A restoration of the appendages drawn by Doctor Elvira Wood from the original specimens and from the photographs made by Professor Beecher. × 9.
The appendages of the thorax and pygidium can fortunately be taken quite directly from the photographs of the dorsal and ventral sides of well preserved specimens. Thereis of course a question as to the number and the exact form of those on the pygidium, but I think the present restoration is fairly well justified by the specimens. As would be expected from the narrow axial lobe, the gnathobases of the coxopodites are short and small.
COMPARISON OF APPENDAGES OF DIFFERENT GENERA.
Since the appendages ofTriarthrus,Cryptolithus,Neolenus,Calymene, andCeraurusare now known with some degree of completeness, those ofIsotelussomewhat less fully, and something at least of those ofPtychoparia,Kootenia, andAcidaspis, these forms being representatives of all three orders and of seven different families of trilobites, it is of some interest to compare the homologous organs of each.
All in which the various appendages are preserved prove to have a pair of antennules, four pairs of biramous limbs on the cephalon, as many pairs of biramous limbs as there are segments in the thorax, and a variable number of pairs on the pygidium, with, in the case ofNeolenusalone, a pair of tactile organs at the posterior end. Each limb, whether of cephalon, thorax, or pygidium, consists of a coxopodite, which is attached on its dorsal side to the ventral integument and supported by an appendifer, an exopodite, and an endopodite. The exopodite is setiferous, and the shaft is of variable form, consisting of one, two, or numerous segments. The endopodite always has six segments, the distal one armed with short movable spines.
Coxopodite.
The coxopodite does not correspond to the protopodite of higher Crustacea, the basipodite remaining as a separate entity. The inner end of the coxopodite is prolonged into a flattened or cylindrical process, which on the cephalon is more or less modified to assist in feeding, and so becomes a gnathobase or gnathite. The inner ends of the coxopodites of the thorax and pygidium are also prolonged in a similar fashion, but are generally somewhat less modified. These organs also undoubtedly assisted in carrying food forward to the mouth, but since they probably had other functions as well, I prefer to give them the more non-committal name of endobases.
InTriarthrusandNeolenusthe endobases are flattened and taper somewhat toward the inward end. InIsotelus,CalymeneandCeraurus, they appear to have been cylindrical. In other genera they are not yet well known. In all cases, particularly about the mouth, they appear to have been directed somewhat backward from the point of attachment. As it is supposed that these organs moved freely forward and backward, the position in which they occur in the best preserved fossils should indicate something of their natural position when muscles were relaxed.
Cephalon.
Antennules.—Antennules are known inTriarthrus,Cryptolithus,Neolenus, andPtychoparia. In all they are long, slender, and composed of numerous segments, which are spiniferous inNeolenus, and very probably so in the other genera.
InTriarthrus,Neolenus, andPtychopariathey project ahead of the cephalon, emerging quite close together under the front of the glabella, one on either side of the median line. InCryptolithusthey turn backward beneath the body, but since only three or four specimens are known which retain them, it is possible that other specimens would showthat these organs were capable of being turned forward as well as backward. The proximal ends of the antennules being ball-like, it is probable, as Doctor Faxon has suggested to me, that these "feelers" had considerable freedom of motion. The antennules ofTriarthrusare apparently somewhat less flexible than those of the other genera, and have a double curvature that is seen among the others only in Ptychoparia. The proximal end of an antennule inTriarthrusis a short cylindrical shaft, apparently articulating in a sort of ball-and-socket joint. The proximal end in the other genera is still unknown. The points of attachment inTriarthrusseem to be under the inner part of the second pair of glabellar furrows. InCryptolithusthey appear to be beside the anterior lobe of the glabella under what have long been known as the antennal pits. In the other genera the location is not definitely known, but inNeolenusit seems to be under the dorsal furrows near the anterior end of the glabella. Viewed from the under side, the point of attachment is probably always beside the middle or anterior part of the hypostoma, just behind the side wings.
Paired biramous appendages.—Behind the antennules all the appendages except those on the anal segment are biramous, consisting of a coxopodite with an inward-directed endobase and an outward-directed pair of branches, the exopodite above, and the six-jointed endopodite beneath. The basipodite really bears the exopodite, but the latter also touches the coxopodite. This structure has been seen inTriarthrus,Cryptolithus,Neolenus,Kootenia,Calymene,Ceraurus, andPtychoparia. InTriarthrus,Neolenus,Acidaspis,Ptyclioparia, and Kootenia, the appendages extend beyond the margins of the dorsal test. InCryptolithusandIsotelusnone (other than antennules) does so. InIsotelusandAcidaspisonly the endopodites have been seen. InTriarthrus,Calymene,Ceraurus, andNeolenusthere are four pairs of appendages behind the antennules. The other genera probably had the same number, but the full structure of the under part of their cephala is not known. InTriarthrusthe endopodites of the cephalon are slender, the individual segments parallel-sided, the inner ones flattened, the outer ones cylindrical in section. They project slightly beyond the edge of the cephalon when fully extended, and each terminates in three small spines. InCryptolithusthe endopodites of the cephalon are longer than those of the thorax, but with the possible exception of the first pair, are bent backward at the carpopodite, and do not ordinarily project beyond the brim of the test. InNeolenusthe endopodites of the cephalon are rather thick and wide, but are long, project forward, and extend beyond the brim. The individual segments are flattened, probably compressed oval in section. The terminal segment of each is furnished with three strong spines at its distal end. InCalymeneandCeraurusthe endopodites appear to consist of slender segments which are oval or circular in section. InCalymeneWalcott believed the three distal segments of the last endopodites of the head to be greatly enlarged, giving these appendages a paddle-like form similar to some of the appendages of eurypterids. The evidence for this does not seem to me to be good. The cephalic endopodites ofIsotelusare entirely similar to those of the thorax, and are rather short, consisting of a series of short cylindrical segments which do not taper greatly toward the distal end. The endopodites of the cephalon ofAcidaspis,Kootenia, andPtychopariaare still unknown.
The exopodites of the cephalon seem in all known cases (Triarthrus,Cryptolithus,Neolenus, and Ceraurus) to be like those of the thorax. They point more directly forward in most cases, project beyond the margin of the head normally only in Triarthrus, and usually occupy the region under the cheeks (fixed and free).
The endobases of the coxopodites of the appendages of the cephalon probably in all casesfunction as mouth-parts (gnathites), and are especially modified for this purpose in Triarthrus, being flattened, shoe-shaped in outline, and so arranged that they work over one another in a shearing fashion. While the more anterior of the coxopodites are attached in front of the posterior tip of the hypostoma, the gnathites of Triarthrus bend backward so that all are behind the hypostoma. InCalymeneandCeraurus, two or three pairs of the gnathites are back of the hypostoma, and one or more pairs may be beside or under the hypostoma. In these genera the mouth is probably in front of the tip of the upper lip. InIsotelus, the mouth seems to have been situated in the notch between the two branches of the hypostoma, and the gnathites of two or three pairs of the appendages probably worked under its forks. Since the length of the hypostoma differs in the various species ofIsotelus, there would be a variable number of gnathites projecting under its forks, according to the species. In this genus the gnathites are of the same long form, cylindrical in cross-section, as the endobases of the thoracic segments, but each is bowed back considerably from the point of attachment.
The gnathites ofNeolenusare like the endobases of the thorax, but broader. The great length of the hypostoma makes it probable that the mouth was far back and that some of the gnathites were in front of it. The gnathites ofCryptolithusare unknown. Professor Beecher in his drawing shows some fragments with toothed ends near the hypostoma, and it may be that they are inner ends of gnathites, but I see nothing to substantiate such an interpretation. If, as some suppose,Cryptolithuswas a mud feeder, the gnathites were probably poorly developed. Of the gnathites ofKootenia,Ptychoparia, andAcidaspisalso nothing is known.
Thorax.
In each genus there is a pair of appendages for each segment of the thorax. When the axial lobe is narrow, the endobases of the coxopodites are small and short (Cryptolithus,Ceraurus,Calymene). When the axial lobe is wide, the endobases are long and stout (Isotelus,Triarthrus). The exopodites always lie above and in front of the corresponding endopodites. In Triarthrus the two branches are of practically equal length. InCryptolithusthe exopodites are much the longer. InNeolenus,Calymene,Ceraurus,Kootenia, andPtychoparia, the exopodites are shorter than the endopodites.
The exopodites inTriarthrusconsist of a proximal shaft, succeeded by numerous short segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along the anterior margin of the shaft there are many small spines. Along the posterior margin there are numerous flattened setæ, which all lie in one plane and which seem to be more or less united to one another like the barbs of a feather. The setæ are short, not much longer than the width of one of the thoracic segments, and point backward and outward. InCryptolithusthe shaft does not seem to be made up of small segments, and is narrow, with a decided backward curve. The setæ are considerably longer and much more flattened than in Triarthrus. InCalymenethe state of preservation does not allow a very full knowledge of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate setæ. The coiled branches of the exopodites as described by Walcott seem to me to be only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the view of Beecher. InCeraurusthe exopodite seems to have been somewhat paddle-shaped, expanded at the distal end, and to have had rather thick, blade-like setæ.
The exopodite ofNeolenusis decidedly leaf-like, and reminds one somewhat of the exitesof some of the phyllopods. The shaft is a broad unsegmented blade. The setæ are slender, delicate, flattened, and a little longer than the width of the shaft. The exopodites of this genus point forward all along the body. InKooteniathe exopodites are like those ofNeolenus, but with a narrower shaft. The exopodites ofPtychopariaappear to be very much like those ofTriarthrus, but the shaft is probably not segmented.
The endopodites of the thorax ofTriarthrus,Cryptolithus, andAcidaspisshow progressive modification from front to back in the broadening of the individual segments and the assumption by them of a triangular form. Not only do the individual segments become more triangular from front to back, but more of the segments of each endopodite become triangular. This modification has so far been seen in these three genera only. The individual segments, except the distal ones, seem to be flattened in all these genera. The distal end of the terminal segment of each endopodite ofTriarthrusbears three small movable spines, and each of the segments usually bears three or more spines, located in sockets along the dorsal surface and at the anterior distal angle of each segment. The endopodite ofCryptolithusis bent backward at the carpopodite and this segment is always thickened. At the distal end of the dactylopodite there is a tuft of spines, the triangular segments have tufts of spines on their posterior corners, and there are groups of spines also in the neighborhood of the articulations.
The endopodites ofCeraurus,Calymene, andIsotelusare all relatively slender, the segments are parallel-sided, and there seems to be no particular modification from front to back of the thorax. The endopodites ofIsotelusare short, the entire six segments of one being but little longer than the coxopodite of the same appendage. The segments of the endopodites ofNeolenusare mostly short and wide, and at the distal end of the terminal segment there are three stout spines. InKooteniathe endopodites are long and very slender. The endopodites of Ptychoparia are too poorly preserved to show details, and those of the thorax ofAcidaspislikewise reveal little structure, but they seem to have the triangular modification, and to turn back somewhat sharply at about the position of the carpopodite.
Pygidium.
Beecher showed that inTriarthrusthere was a pair of appendages on the pygidium for every segment of which it is composed except the last or anal segment (protopygidium). Walcott has since shown that inNeolenusthis segment bears a pair of cerci, and Beecher's drawings show that in his later studies he recognized a spinous plate, the possible bearer of cerci, on the anal segment ofTriarthrus. The appendages of the anal segment have not yet been seen on other species of trilobites.
The appendages of the pygidium do not show any special modifications, but seem in all cases to be similar to those of the posterior part of the thorax. InCryptolithusall the pygidial appendages are short and remain beneath the cover of the dorsal test, while inTriarthrusandNeolenusthey extend behind it.
In the latter genus the endopodites of the pygidial appendages appear to be practically identical in form with those of the thorax, the individual segments being perhaps a little more nearly square in outline. Like those of the thorax, the segments of the pygidial endopodites bear numerous short spines. The caudal cerci are richly segmented, slightly flexible, spinous tactile organs. They are symmetrically placed, nearly straight when in their natural position, and make an angle of about 75 with one another. They appear to beattached to a narrow rim-like plate which seems to fit in just ahead of the doublure of the pygidium, or perhaps over it.
InCeraurus,Calymene, andIsotelus, the endopodites of the pygidium are similar to those of the thorax, but seemingly more slender, with less well developed coxopodites, and with, in the last-named genus, slender cylindrical segments. Exopodites are not known on the pygidia of any of these genera, but since they are present and like those of the thorax inTriarthrus,Cryptolithus,Neolenus, andPtychoparia, there is little reason to think that they were absent inCeraurusorCalymene, though there is some question aboutIsotelus.
The limbs are largest and longest on the anterior part of the thorax of a trilobite, and diminish regularly in length and strength to the posterior end of the pygidium. This regular gradation shows, as Beecher was the first to point out, that the growing point of the trilobites is, as in other arthropods, in front of the anal segment. Newfreesegments are introduced into the thorax at the anterior end of the pygidium, and this has led to some confusion between the growing point and the place of introduction of free segments.
If a new segment were introduced at a moult in front of the pygidium, that segment would probably have less fully developed appendages than those adjacent to it, and so make a break in the regular succession. The condition of the appendages corroborates the evidence derived from the ontogeny of the pygidium, and proves that the new segments are introduced at the same growing point as in other Arthropoda.
Caudal Rami.
Bernard, who believed that the Crustacea had been derived through anApus-like ancestor (1892, pp. 20, 85, 274), pointed out that four or less than four anal cirri were to be expected. Two well developed cirri and two rudimentary ones are present inApus, and they are also to be found in other phyllopods and some isopods. It is, however, characteristic of the Crustacea as a whole to lack appendages on the anal segment. Caudal cirri (cerci) are much more freely developed in the hexapods than in the Crustacea, particularly in the more primitive orders, Palæodictyoptera, Apterygota, Archiptera, and Neuroptera. They are supposed, in this case, to be modified limbs, and therefore not homologous with the bristles on the anal segment of an annelid. Doctor W. M. Wheeler of the Bussey Institution has kindly allowed me to quote the following excerpt from a letter to me, as expressing the opinion of one who has made an extensive study of the embryology of insects:
I would say that I have no doubt that the cerci of insects are directly inherited from the insect ancestors. They are always highly developed in the lower insects, and only absent or vestigial in a few of the most highly specialized orders such as the Hemiptera, Diptera, and Hymenoptera. I have further no doubt concerning their being originally ambulatory in function. They are certainly not developed independently in insects. Embryologically they arise precisely like the legs, and each cercus contains a diverticulum of the mesoblastic somite precisely as is the case with the ambulatory legs and mouth parts.
I would say that I have no doubt that the cerci of insects are directly inherited from the insect ancestors. They are always highly developed in the lower insects, and only absent or vestigial in a few of the most highly specialized orders such as the Hemiptera, Diptera, and Hymenoptera. I have further no doubt concerning their being originally ambulatory in function. They are certainly not developed independently in insects. Embryologically they arise precisely like the legs, and each cercus contains a diverticulum of the mesoblastic somite precisely as is the case with the ambulatory legs and mouth parts.
The "pygidial antennæ" seem to be as fully developed inNeolenusas in any of the other arthropods, and may suggest a common ancestry of the phyllopods, isopods, and hexapods, in the trilobites. They were doubtless tactile organs, and while the evidence is chiefly negative, it would seem that they proved useless, and were lost early in the phylogeny of this group. Possibly the use of the pygidium as a swimming organ proved destructive to them.
HOMOLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA.
The head of the typical crustacean bears five pairs of appendages, namely, the antennules, antennas, mandibles, and first and second maxillæ, or, as they are more properly called, the maxillulæ and maxillæ.
As Beecher has pointed out, the "antennæ" of the trilobites, on account of their pre-oral position and invariably uniramous character, are quite certainly to be correlated with the antennules.
The second pair of appendages, the first pair of biramous ones, Beecher homologized with the antennæ of other crustaceans, and that homology has been generally accepted, though Kingsley (1897) suggested that it was possible that no representatives of the true antennæ were present.
In preparing the restorations in the present study, the greatest difficulty has been to adjust the organs about the mouth. InTriarthrus, numerous specimens show that without question there are four pairs of gnathites back of the hypostoma, and that all four belong to the cephalon. In forms with a long hypostoma, however, there was no room on the cephalon for the attachment of four pairs of gnathites, neither were there enough appendifers to supply the requisite fulcra. At first I supposed I had solved the difficulty by assuming the mouth to be in front of the posterior tip of the hypostoma, as it really is in Ceraurus andCalymene, and allowing the gnathites to play under the hypostoma as Walcott (1912) has shown that they do inMarrella. Finally, when I came to study in greater detail the slices ofCalymeneandCeraurus, they seemed to show that the anterior one or two pairs of appendages became degenerate and under-developed. This was probably a specialization due to the great development of the hypostoma in trilobites, that organ being much more prominent in this than in any other group. As the hypostoma lengthened to accommodate the increasing size of sub-glabellar organs (stomach, heart, etc.), the mouth migrated backward, leaving the anterior appendages ahead of it, with their gnathobases, at least, functionless. That such migration has taken place, even in Triarthrus, is shown by the fact that the points of articulation of the first biramous appendages are pre-oral, and it is more obviously true ofCeraurus. Correlated with the weakening of the appendages on the lower surface is the loss of glabellar furrows on the upper surface. The glabellar furrows mark lines of infolding of the test to form the appendifers and other rugosities for the attachment of tendons and muscles. It is conceivable that this migration backward of the mouth began very early in the history of the race, and that even before Cambrian times, the antennæ, probably originally biramous appendages like those on the remainder of the body, had dwindled away and become lost. If this is the case, then the first pair of biramous appendages ofTriarthruswould be mandibles, the second pair maxillulæ, and the third pair maxillæ.
There remain the last pair of cephalic appendages, and they bring up the whole head problem of the trilobites. Beecher has stated (1897 A, p. 96) his conviction that the head of the trilobite is made up of five segments, representing the third, fourth, fifth, sixth, and seventh neuromeres of the theoretical crustacean. As a matter of fact, he really made up the head of seven segments, since he stated that the first neuromere was represented by the hypostoma and the second by the epistoma and free cheeks.
Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight segments, as he sawfive segments in the glabella of certain trilobites. In his table (p. 165) he has listed the segments with their appendages as follows: 1. Acron, with hypostoma; 2, rostrum (epistoma), with free cheeks; 3, first frontal lobe, with (?) antennules; 4, second frontal lobe, with antennæ; 5, mandibles; 6, first, or pre-maxillæ; 7, second maxillæ; 8, occipital segment with maxillipeds.
Jaekel refused to believe that the antennæ of trilobites were really entirely simple, and so homologized them with the antennæ and not the antennules of other Crustacea. In this he was obviously incorrect, but it accounts for his homology 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 (Mesonacidæ, Paradoxidæ Cheiruridæ, 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 inMarrella, where a trilobite-like cephalon bears five pairs of appendages, the second pair of which are tactile antennæ, is favorable to the above interpretation. In spite of the apparent degeneration of the first two pairs of appendages inCalymene, no limbs are actually missing, and if some are dropped out in the later trilobites it would not affect the homology of those now known. I therefore agree with Beecher in homologizing 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 ofCryptolithusretaining 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 setæ are particularly adapted. Thatthey 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 setæ 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.
AlthoughNeolenusis usually accounted a less primitive form thanPtychopariaorTriarthrus, it has much the most primitive type of exopodite yet known. It would appear that the exopodites were originally broad, thin, simple lamellæ, which became broken up, on the posterior side, into fine cylindrical setæ. 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 setæ in turn became modified from their original simple cylindrical shape to form the wide, thin, blade-like filaments ofCryptolithusandCeraurus.
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 ofCryptolithus, and probably others, for burrowing; and probably in all for gathering food, in which function the numerous spines with which they are arrayed doubtless assisted.
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 theProtichnitesrecently 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 partof the thorax and pygidium inTriarthrus,Cryptolithus, andAcidaspisprobably 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 likeIsotelus, 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 ofCryptolithus, combined with its shovel-shaped cephalon, indicateLimulus-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 principal 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 Agnostidæ, Asaphidæ, Phacopidæ, 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 Harpedidæ and Cheiruridæ 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 Goldiidæ and Lichadidæ. 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 preservedDipleurain this position is on exhibition in the Museum of Comparative Zoology, and Mr. Narraway and I have figured aBumastus milleriin the same attitude (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 mass 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 Agnostidæ and Eodiscidæ, all Lower Cambrian trilobites had small pygidia, and the same is true of those of the Middle Cambrian of the Atlantic realm (except for theDorypygeof Bornholm). In Pacific seas, however, large-tailed trilobites of the families Oryctocephalidæ, Bathyuridæ, and Asaphidæ 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 Agnostidæ were the sole representatives of the isopygous trilobites, while in the Pacific still another family, the Dikelocephalidæ, 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. Asaphidæ and Illænidæ were then dominant, and the Proëtidæ, Cyclopygidæ Goldiidæ, and Lichadidæ had begun their existence. A similar story is told by the Silurian record, except that the burden of the Asaphidæ has been transferred to the Lichadidæ and Goldiidæ. All the really old (Cambrian) families of trilobites with small pygidia had now disappeared. In the general dwindling of the subclass through the Devonian and later Palæozoic, the few surviving species with small pygidia were the first to go, and the proëtids 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.
Coxopodites.
The primary function of the endobases of the coxopodites 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 pass 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 coxopodites 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 ofIsotelusand probably other trilobites with wide axial lobes was that of locomotion. InIsotelusthe 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 inTriarthrusandCryptolithus, and apparently slight or not effective inIsotelus,Ceraurus, andCalymene. These last are, however, from the conditions of preservation, least available for study.
InPart 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.Neolenushad 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 attitude was adopted by trilobites has been shown by Walcott in his explanation of the trails known asProtichnites(1912 B, p. 278). Many trilobites probably crawled inthis 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 ofTriarthrus, 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.
Isotelushas endopodites different from those of eitherNeolenusorTriarthrus. 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 thatIsotelus, like Neolenus, crawled on its "toes." The endopodites ofIsotelusare however, short and feeble when compared with the size of the test, while the endobases of the coxopodites are extraordinarily developed. These facts, together with certain trails, strongly suggest the use of the coxopodites as the primary ambulatory organs, the endopodites probably assisting. In this event, the position of the endopodites and coxopodites 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 coxopodites 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 coxopodites and endopodites of all the trilobites with wide axial lobes,Nileus,Bumastus,Homalonotus, etc., were developed in this same way.
Cryptolithuspresents 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 ofLimulus.
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 nothingis as yet known of the musculature. The coxopodites 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 coxopodites 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 setæ gracefully pendent, but pendent as a sheet and not individually, there having been some method by which adjoining setæ 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.