The Appendages, Anatomy, and Relationships of Trilobites Part 7

All of the work on these species has been done by Doctor Walcott, who summarized his results in 1881.

In the first of his papers (1875, p. 159), Walcott did not describe any appendages but paved the way for further work by a detailed and accurate description of the ventral surface of the dorsal sh.e.l.l of Ceraurus. He demonstrated the presence in this species of strongly b.u.t.tressed processes which extend directly downward from the test just within the line of the dorsal furrows. One pair of these is seen beneath each pair of the glabellar furrows, each segment of the thorax has a pair, and there are four pairs on the pygidium. He pointed out also that these projections were but poorly developed on that part of the glabella which is covered by the hypostoma. He called them axial processes, the only name which appears to have been suggested thus far.

The first announcement of the discovery of actual appendages in _Ceraurus_ and _Calymene_ was made by the same investigator in a pamphlet published in 1876 in advance of the 28th Report of the New York State Museum of Natural History, the publication of the whole report being delayed till 1879. The results were obtained by the process of cutting translucent slices of enrolled trilobites derived from the Trenton limestone at Trenton Falls, New York. Since he summarized all the results of this study in one paper at a later date, it is not necessary to follow the stages of the work.

A second preliminary paper was published in pamphlet form in September, 1877, and in final form in 1879, when the first figures were presented.

In his important paper of 1881, Walcott reviewed all that was known of the appendages of trilobites to that time, and gave the results of seven years of study of sections of enrolled specimens. Slices had been made of 2,200 individuals from Trenton Falls, which resulted in obtaining 270 which were worthy of study. Of these, 205 were from _Ceraurus pleurexanthemus_, 49 from _Calymene senaria_, 11 from _Isotelus gigas_, and 5 from _Acidaspis trentonensis_.

Walcott's views on certain portions of the anatomy can best be set forth in the form of a few extracts (1881, pp. 199-208):

_The Ventral Membrane._--In those longitudinal sections in which the ventral membrane is most perfectly preserved, it is shown to have been a thin, delicate pellicle or membrane, strengthened in each segment by a transverse arch, to which the appendages were attached. These arches appear as flat bands separated by a thin connecting membrane, somewhat as the arches in the ventral surface of some of the Macrouran Decapods....

In by far the greater number of sections, both transverse and longitudinal, the evidence of the former presence of an exterior membrane, protecting the contents of the visceral cavity, rests on the fact that the sections show a definite boundary line between the white calcspar, filling the s.p.a.ce formerly occupied by the viscera, and the dark limestone matrix. Even the thickened arches are rarely seen.

The mode of attachment of the leg to the ventral surface is shown [in transverse and longitudinal sections of _Ceraurus_ and _Calymene_].

These ill.u.s.trations are considered as showing that the point of articulation was a small, round process projecting from the posterior surface of the large basal joint, and articulating in the ventral arch somewhat as the legs of some of the Isopods articulate with the arches in the ventral membrane. The arches of the ventral membrane in the trilobite ... afford a correspondingly firm basis for the attachment of the legs.

Branchial appendages.--The branchiae have required more time and labor to determine their true structure than any of the appendages yet discovered. They were first regarded as small tubes arranged side by side, like the teeth in a rake; then as setiferous appendages, and finally as elongate ribbon-like spirals and bands attached to the side of the thoracic cavity, the epipodite being a so-called branchial arm.

All of these parts are now known to belong to the respiratory system, but from their somewhat complex structure, and the various curious forms a.s.sumed by the parts when broken up and distorted, it was a long time before their relations were determined.

The respiratory system is formed of two series of appendages, as found beneath the thorax. The first is a series of branchiae attached to the basal joints of the legs, and the second, the branchial arms, or epipodites.

The branchiae, as found in _Calymene_, _Ceraurus_, and _Acidaspis_, have three forms. In the first they bifurcate a short distance from the attachment to the basal joint of the leg, and extend outward and downward as two simple, slender tubes, or ribbon-like filaments.

In the second form they bifurcate in the same mariner, but the two branches are spirals. These two forms occur in the same individual but, as a rule, the more simple ribbon-like branchia is found in the smaller or younger specimens, and the spiral form in the adult.... The spiral branchiae of Ceraurus are usually larger and coa.r.s.er than those of _Calymene_.

The third type of the branchiae [consists of rather long straight ribbons arranged in a digitate manner on a broad basal joint]. As far as yet known, this is confined to the anterior segments of the thorax.

The epipodite or branchial arm was attached to the basal joints of the thoracic legs and formed of two or more joints. This has been called a branchial arm, not that it carried a branchia, but on account of its relation to the respiratory system. It is regarded as an arm or paddle, that, kept in constant motion, produced a current of water circulating among the branchiae gathered close beneath the dorsal sh.e.l.l... .

Of the modification the respiratory apparatus underwent beneath the pygidium, we have no evidence.

In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott has reviewed his earlier work on _Calymene_ and _Ceraurus_, and presented a new restoration of the former. The c.o.xopodites are now interpreted as being similar to those of _Triarthrus_ and Neolenus, but the exopodites are still held to be spiral and the setiferous organs labelled as epipodites rather than exopodites.

Comparison of the Appendages of Calymene and Ceraurus with those of Triarthrus.

As one may see by reading the above quotations from Doctor Walcott's descriptions, he found certain branchial organs in _Ceraurus_ and _Calymene_ which have not been found in other trilobites but otherwise the essential features of the appendages of all are in agreement.

Spiral Branchiae.

It is now necessary to inquire if the thin sections can not be interpreted on the basis of trilobites with the same organs as _Triarthrus_. The interpretation of the structures seen in these translucent slices is exceedingly difficult, and Doctor Walcott deserves the utmost praise for the ac.u.men with which he drew his deductions. Even with the present knowledge of _Triarthrus_, _Isotelus_, and _Neolenus_ as a guide, I do not think it is safe to speak dogmatically about what one sees in them.

Walcott has summarized his results in his restoration of the appendages of _Calymene_ (1918, pl. 33). The c.o.xopodite supports a slender six-jointed endopodite as in _Triarthrus_, dorsal to which is a short setiferous epipodite which differs from the exopodite of _Triarthrus_, in being less long, unsegmented, and in having shorter setae. Arising from the same part of the c.o.xopodite with this epipodite is the bifurcate spiral branchia which has not been seen in this form in other trilobites. The evidence on which the existence of this organ is postulated consists of a series of sections across the thorax, the best of them figured by Walcott in his plates 2 and 3 (1881) and plate 27 (1918).

The specimens sliced were all partially or quite enrolled, and in that position one would expect to find the appendages so displaced that it would be only rarely that a section would be cut, either by chance or design, in such a direction as to show any considerable part of any one appendage. This expectation has proved true in regard to the endopodites, the sections rarely showing more than two or three consecutive segments. Sections like those shown in figures 1 and 2 in plate 2 (1881) seem to be unique. On the other hand, there are numerous slices showing the so-called spiral branchiae. They show for the most part as a succession of rectangular to kidney-shaped spots of clear calcite.[1] Usually these clear spots are isolated, not confluent, but in a small number of specimens, perhaps three or four, the spots are connected in such a way as to show a zig-zag band which suggests a spiral. Such an explanation is of course entirely reasonable, but it would be surprising if so slender a spiral should be cut in such a way as to exhibit the large series of successive turns shown in many of these thin sections. Continuous sections of such organs should be no more common than continuous sections of endopodites.

[Footnote 1: In looking at Walcott's figures of 1881, it should be remembered that the dark portions of the figures are clear calcite in the specimens, while the light part is the more or less opaque matrix.]

One of the arguments against the interpretation of these series of spots as sections across spiral arms is that of probabilities. It is known from flattened specimens that _Neolenus_, _Kootenia_, _Ptychoparia_, _Triarthrus_, and _Cryptolithus_ all have a single type of exopodite, consisting of a simple setiferous shaft. All these genera have been examined in a way that permits no doubt about the structure, and no trace of spiral arms has been detected. On the other hand, Walcott found spiral arms in three unrelated genera, _Calymene_, _Ceraurus_, and _Acidaspis_, all of the trilobites in which he found exopodites by the method of sectioning. What are the probabilities that genera of three different families, studied by means of sections, should agree in having a type of exopodite different from that of the five genera about whose interpretation there can be no doubt?

Another argument against the interpretation of the sections as spirals is that in any one line the individual spots are of roughly uniform size. This means of course that the spiral has been cut by a plane parallel to the tangent plane. This might happen once, just as once Doctor Walcott cut all six segments of a single endopodite, but that it should happen repeatedly is highly improbable. Moreover, there is a limit to the diameter of the section which may be made from these slender spirals. Most of the spots have one diameter about one half greater than the other, but others are from three to six times as long as wide. These last could obviously be cut only from a very large spiral, and they are therefore interpreted by Walcott as setae of epipodites. Yet all gradations are found among the sections, from the long setae to the short dots. (See pl. 27, 1918.) In referring to one slice, Walcott says (1918, p. 152):

In the latter figure and in figure 13, plate 27, the setae of several epipodites appear to have been cut across so as to give the effect of long rows of setae. The same condition occurs in specimens of _Marrella_ when the setae of several exopodites are matted against each other.

[Ill.u.s.tration: Fig. 12.--A slice of _Ceraurus pleurexanthemus_ in which the exopodite happened to be cut in such a way as to show a part of the shaft and some of the setae in longitudinal section. Specimen 80. 4.]

This is certainly an apt comparison, and equally true if _Neolenus_, _Triarthrus_, or _Cryptolithus_ were subst.i.tuted for _Marrella_.

Now consider the "epipodites." They are well shown in _Calymene_ in the specimens ill.u.s.trated on plate 27, figure 11 (1918), and plate 3, figure 3 (1881), and less clearly in one or two others. Slices 22 (pl.

27, fig. 12, 1918) and 80 (our fig. 12) show what is called the same organ in Ceraurus. It will be noted that all of these slices are cut in the same way, that is, more or less parallel to the under surface of the head, or, at any rate, on a plane parallel to a plane which would be tangent to the axial portion of the coiled sh.e.l.l. The sections which show the spirals best are those which are cut by a plane perpendicular to the long axis of the body. If one were to attempt to cut an enrolled _Triarthrus_ in such a way as to get a section showing the length of the setae, one would not cut a section perpendicular to the axis of the animal, nor, in fact, would he cut one parallel to the ventral plane, but it is obvious that in this latter type of section he would stand a better chance of finding a part of the plane of the exopodite coincident with the plane of his section than in the former. And that seems to be what has happened in these sections of _Calymene_ and _Ceraurus_. If the exopodites were preserved, transverse sections were bound to cut across many sets of fringes, and the resultant slice would show transverse sections of the setae as a series of overlapping spots. A few fortunately located sections in a more nearly horizontal plane might cut the setae and occasionally the shaft of one or more exopodites in the longitudinal plane, and the resulting effect would produce the so-called "epipodites." A careful study has shown that no one of these epipodites is complete, and they do not have the palmate form shown in Walcott's figures.

And the last and most important argument against the spiral appendages is that certain slices, of both _Calymene_ and _Ceraurus_, show definitely exopodites of exactly the type found in other trilobites.

These are discussed later in the detailed description of the various slices.

If these series of spots are interpreted on the basis of the known structure of _Triarthrus_, they are of course a series of sections through the setae of the exopodites. It will be shown in Part IV that these setae are not circular in section, but flattened, in _Cryptolithus_ even blade-like, and that they overlap one another. A section across them would give the same general appearance as, for instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 3 (1881).

When both endopodites and the "spiral branchiae" are present in the same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiae"

are dorsal to the endopodites, as the setae of the exopodites would be expected to be. The specimens which show the clear spots connected, and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to bear evidence against this interpretation, but one has only to think of the effect of cutting a section along the edge where the setae are attached to the shaft of the exopodite of _Triarthrus_ to see that such a zig-zag effect is entirely possible. One would expect to cut just this position only rarely, and, in fact, the zig-zags are seen in only three or four sections. The bifurcation of the basal segment of the "spiral branchiae" (pl. 3, fig. 10, 1881) is probably more apparent than real, if indeed these basal segments have anything to do with the succeeding one.

A second peculiarity of _Calymene_, shown in Walcott's restoration, is the great enlargement of the c.o.xopodites and of the distal segments of the endopodites of the fifth pair of appendages of the cephalon. This is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881).

After a study of the specimens I regret to find myself still unconvinced that the posterior cephalic appendages were any larger than those in front.

Ventral Membrane.

The most striking value of the thin sections of _Ceraurus_ and _Calymene_, and therein they have a great superiority over all the other forms so far investigated, is that they show the extent of the body cavity and the position, though not the substance, of the ventral membrane. Transverse sections through _Ceraurus_ (Walcott's pl. 1.

figs. 1-5; pl. 2, figs. 1, 3, 1881) and _Calymene_ (pl. 3, figs. 9, 10, 1881) show that the body cavity was almost entirely confined to the axial lobe. The longitudinal sections of _Ceraurus_ (pl. 2, figs.

6, 8; pl. 4, fig. 8) and of _Calymene_ (pl. 2, figs. 5, 7; pl. 5, figs. 1-4) show that the ventral membrane was exceedingly thin and was wrinkled transversely when the sh.e.l.l was enrolled.

The specimens of figures 1-3, plate 5 (1881) show the form of the ventral membrane more distinctly than any of the others. The section of figure 1 was cut just inside the dorsal furrow on the right side, and figure 2, which is on the opposite side of the same slice, is almost exactly on the median line. Figure 3 shows a section just inside the left dorsal furrow. Section 2 did not cut any of the appendages, and the ventral membrane is shown as a thickened, probably chitinous sheet thrown into low sharply crested folds equal in number to, and pointing in a direction just the reverse of, the crests of the segments of the thorax. Under the pygidium, where there would of course be less wrinkling, the folds are hardly noticeable. In the actual specimens one sees more plainly than in the figures the line of separation between the ventral membrane and the appendages, but the state of preservation of everything beneath the dorsal sh.e.l.l is so indefinite that one does not feel sure just what the connection between the appendages and the membrane was. In the original of figure 5, plate 2, which seems to have been cut so as to cross the appendages at their line of junction with the ventral membrane, there appear to be narrow chitinous (?) plates extending from the ventral membrane to the dorsal test.

Appendifers.

In Ceraurus there are regular calcareous processes which extend down from the dorsal test just inside the line of the dorsal furrow, and which undoubtedly serve as points of attachment of the appendages.

These processes, which for convenience I have designated as "appendifers," are broken off in most specimens showing the lower surface of _Ceraurus pleurexanthemus_, but on certain ones cleaned with potash they are well preserved. Doctor Walcott showed them well in his figures of the lower surface of this species (1875, pl. 11; 1881, pl. 4, fig. 5), while the attempt of Raymond and Barton (1913, pl. 2, fig. 7) to show them by photography was not so successful.

There is one pair of appendifers on each of the thoracic segments and four pairs on the pygidium. On the cephalon there is one pair under the neck furrow, and a pair under the posterior glabellar furrows.

These are not concealed by the hypostoma. Further forward, and completely covered by the hypostoma, are two much less strongly developed but similar ones, so that there are in all four pairs of appendifers on the cephalon, though it is extremely doubtful if the appendages were articulated directly to all of them. On a specimen of _Ceraurus pleurexanthemus_ 30 mm. long on the median line, the dorsal furrows are 7.5 mm. apart at the anterior end of the thorax, and the tips of the appendifers of this segment are only 4 mm. apart. Each consists of a straight slender rod with a k.n.o.blike end projecting directly downward from the dorsal test, and supported by a thin calcareous plate which runs diagonally forward to the anterior edge of the segment directly under the dorsal furrow. On the pygidium three pairs of the appendifers have this form, while the fourth pair consist of low rounded tubercles which are concealed by the doublure. These appendifers are probably cut in many of Walcott's sections of Ceraurus, but owing to the state of preservation it is not always possible to determine what part is appendage, what part is body cavity, and what part is appendifer.

Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10) described and figured the appendifers of Phacops latifrons. He found them to be calcareous projections on the hinder margin of each segment, converging inward, and about 1.5 mm. long. He correctly considered them as supports (Stutzpunkte) for the feet.

Appendifers are well developed also in Pliomerops, and in well preserved specimens of _Calymene senaria_ from Trenton Falls they are present, but instead of being rod-like processes, they are rather thick, prominent folds of the sh.e.l.l. They are also well shown in some of the thin sections. A specimen of _Triarthrus_ (No. 229, our pl. 5, fig. 2) has broad processes extending downward from the lower side of the test below the dorsal furrows, much as in _Calymene_, and the individual of _Cryptolithus_ shown in plate 8, figure 1, possesses slender appendifers. Two other specimens (Nos. 237 and 242) show them quite well. They were probably present in all trilobites, but seldom preserved. The appendifers have the same origin as the entopophyses of _Limulus_, and like them, may have relatively little effect on the dorsal surface.

_Calymene senaria_ Conrad.