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 Branchiæ.
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 acumen 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 coxopodite 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
setæ. Arising from the same part of the coxopodite 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 branchiæ. 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 setæ of
epipodites. Yet all gradations are found among the sections, from the
long setæ 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 setæ of several
epipodites appear to have been cut across so as to give the effect
of long rows of setæ. The same condition occurs in specimens of
_Marrella_ when the setæ of several exopodites are matted against each
other.
[Illustration: 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 setæ in longitudinal section. Specimen
80. × 4.]
This is certainly an apt comparison, and equally true if _Neolenus_,
_Triarthrus_, or _Cryptolithus_ were substituted for _Marrella_.
Now consider the "epipodites." They are well shown in _Calymene_ in
the specimens illustrated 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 shell. 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 setæ, 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
setæ as a series of overlapping spots. A few fortunately located
sections in a more nearly horizontal plane might cut the setæ 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 setæ of the exopodites. It will be shown in Part IV
that these setæ 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 branchiæ" are present in the
same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiæ"
are dorsal to the endopodites, as the setæ 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 setæ 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 branchiæ" (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 coxopodites 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 shell 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 shell
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 knoblike 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 (Stützpunkte) 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 shell. 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.
(Text figs. 13-16, 23.)
Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1,
3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1
(restoration), 2;--Proc. Biol. Soc. Washington, vol. 9, 1894, pl.
1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8,
figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs.
1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_),
13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1
(restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f.
Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann.
Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32;
pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796,
fig. 12.--Bernard, The Apodidæ, 1892, text figs. 50, 52,
54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13,
15, 17.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.
In both of Walcott's accounts (1881, 1918) of the appendages of
_Calymene_ and _Ceraurus_, he has described them together, so that
those who have not taken time to study the illustrations and
disentangle the descriptions are very apt to have a confused notion in
regard to them. I have therefore selected from the original specimens
those slices of _Calymene_ which are most instructive, and bearing in
mind the probable appearance of the appendages of an enrolled
_Triarthrus_, have tried to interpret them. In such a method of study,
I have of course started with a pre-formed theory of what to expect,
but have tried to look for differences as well as likenesses.
_Cephalic Appendages._
_Antennules._--The evidence of antennules rests on a single slice (No.
78). The appendage in question is exceedingly slender and arises at
the side of the hypostoma near its posterior end. It shows fine,
slender segments, and curves first outward and then forward. If it is
in its natural position, it is not an antennule, but the endopodite of
the second or third pair of cephalic appendages. It is short, only
about one-third the length of the hypostoma, but is doubtless
incomplete. The two distal segments show a darker filling, indicating
that they were hollow. Judging from analogy with other trilobites, the
appendage is probably an endopodite and not an antennule. There can be
no reasonable doubt, however, that _Calymene_ possessed antennules.
Some idea of the form of the coxopodites of the cephalic appendages
may be obtained from sections which cut in approximately the plane of
the hypostoma. Such sections are shown in Walcott's photographs (pl.
26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, our fig. 13), 51
(fig. 6), 6 (fig. 11), and 40 (our fig. 14) agree in showing two
pairs of slender coxopodites which are attached at the sides of the
hypostoma and run backward parallel and close to it, and two pairs of
larger coxopodites which are behind the hypostoma, although the point
of attachment of the third pair is in front of its tip. The anterior
pair are apparently under-developed and no longer function as mouth
parts, while the posterior two pairs are large and armed on their
inner ends with spines. Specimen 78, which has already been mentioned
in connection with the antennules, shows a second very slender
appendage back of the so-called antennule, which is equally slender,
but is directed outward instead of forward. It seems not improbable,
from their position and similarity, that these two are the endopodites
of the first two appendages on one side of the hypostoma. Specimen 6
shows rather inadequately the endopodites of the second and third
cephalic appendages. I have not found other slices showing endopodites
of the cephalon. Walcott, in both his restorations, has shown
enlarged, paddle-shaped dactylopodites on the distal ends of the
fourth cephalic endopodites. The evidence for this rests principally
on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12),
and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as
too poorly preserved to be interpreted. No. 53 does show a section of
an appendage which seems to have an unusually wide dactylopodite, but
this slice presents no evidence at all as to the appendage to which
the dactylopodite appertains, nor can one even be sure that there has
not been a secondary enlargement. Specimen 43 shows this feature
much less definitely than is indicated by the published photograph
and drawing. The segment in question is strongly curved, with a
constriction possibly dividing it into two. If it is in its natural
position in this section, it obviously belongs to one of the thoracic
segments and not to the cephalon. With evidence of difference so
unsatisfactory, I prefer to reconstruct the posterior cephalic
endopodites on the same plan as those of the thorax.
[Illustration: Fig. 13.--Slice through _Calymene senaria_ in the plane
of the hypostoma, showing the very slender coxopodites beside that
organ, the spines on the inner end of one of the maxillulæ, and the
anterior position of the attachment of all these appendages. From a
photographic enlargement. Specimen 50. × 4.]
[Illustration: Fig. 14.--Slice through the hypostoma and thorax of
_Calymene senaria_ Conrad, showing the small size of the coxopodites
nearest the hypostoma. Shell in black, appendages and filling of
abdominal cavity dotted. From a photographic enlargement. Specimen 40.
× 3.8.]
[Illustration: Fig. 15.--Transverse section of _Calymene_, showing
method of articulation with the appendifer. The shell is in solid
black, the filling of the appendage and appendifer stippled. Traced
from a photographic enlargement of the slice. Specimen 63. × 7.]
_Exopodites._--Walcott admits that there is no direct evidence of spiral
exopodites in the cephalon of _Calymene_. No one of the sections
cutting through the plane of the hypostoma shows any trace of
appendages which could be interpreted as exopodites.
_Thoracic Appendages._
The large coxopodites of the anterior thoracic appendages are well
shown in many specimens cut longitudinally, of which Nos. 23, 50, and
55 may be mentioned, since photographs of them have been published by
Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward
the proximal ends. Transverse slices show sections of the coxopodites
which are no wider than those in longitudinal sections, indicating
that they were not compressed but probably cylindrical. This is borne
out by an individual (pl. 28, fig. 7, 1918) which is not a slice but
an actual specimen, the body cavity of which was hollow, and, opened
from above, shows the impressions of the last two coxopodites of the
cephalon, and the first four of the thorax.
One transverse section (No. 63, see our fig. 15) is especially
valuable, as it shows the method of articulation of the coxopodites
with the dorsal skeleton. Another specimen (No. 73) shows that
appendifers are present in _Calymene_, and while the appendifer does
not retain its original form in slice No. 63, the section does show
clearly that there was a notch in the inner (upper) side of the
coxopodite into which the lower end of the appendifer fitted, thus
giving a firm, articulated support for the appendage. This notch
appears to be slightly nearer the outer than the inner end of the
coxopodite, and since it must have made a kind of ball-and-socket
joint, considerable freedom of movement was allowed. The appendage
must have been held in place by muscles within the coxopodite and
attached to the appendifer.
No slice which I have seen shows a continuous section through all the
segments of an endopodite, but many, both longitudinal and transverse,
show one, two, or as many as three segments.
Such sections as No. 120 show that the endopodites of the thorax
were slender and composed of segments of rather uniform diameter.
Other sections, notably No. 83, 154, and in, show that they tapered
distally, and bore small spines at the outer end of each segment.
The exopodites of course furnish the chief difficulty in
interpretation. Doctor Walcott finds two sets of structures attached
to the coxopodite, a long, slender, spiral exopodite, and a short,
broad epipodite with a fringe of long setæ. Since he has given the
same interpretation for _Calymene_, _Ceraurus_, and _Acidaspis_, I
have considered the question of all three together on a preceding page
(p. 48), and given my reasons for regarding both structures as due to
sections in different directions across setiferous exopodites.
Sections like those shown in figures 11, 13, and 14 of plate 27 (1918)
happen to be cut in or near the plane of the setæ of an exopodite, and
so show hairs of considerable length. Such sections are, as would be
expected, very few in number, while sections like those shown on
figures 4, 5, 7, and 9 of plate 27, which cut the setæ more nearly at
right angles, are very common. Slices which give any definite idea of
the form of the shaft of the exopodite are exceedingly rare. Perhaps
the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows
the proximal part of a long, slender, unsegmented shaft, with the
bases of a number of slender setæ. The organ is not complete, as would
be inferred from the published figure, but the section cuts diagonally
across it, and the total length is unknown. It is directed forward,
like the exopodites of Neolenus, but whether or not this is a natural
position is yet to be learned.
The proximal, non-setiferous portion of the exopodite is evidently
at an angle with the setiferous part. Another similar exopodite is
apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a
similar angulated shaft and just a trace of the bases of the setæ.
_Pygidial Appendages._
That appendages were present under the pygidium is shown by
longitudinal sections, but nothing is known of the detail of
structure.
[Illustration: Fig. 16. Restoration of _Calymene senaria_ Conrad,
based upon data obtained from the study of the translucent sections
made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the
supervision of the author. About twice natural size.]
_Relation of Hypostoma to Cephalon in Calymene._
In _Calymene_ the shape of the hypostoma bears little relation to the
shape of the glabella, and it is relatively smaller, both shorter and
narrower, than in Ceraurus. In shape, neglecting the side lappets at
the front, it is somewhat rectangular, but rounded at the back, where
it is bifurcated by a shallow notch. The anterior edge has a narrow
flange all across, which is turned at almost right angles to the plane
of the appendage, and which fits against the doublure of the free
cheeks at the sides and against the epistoma in the middle. The side
lappets show on their inner (upper) surface shallow pits, one on each
lappet, which fit over projections that on the dorsal surface show as
deep pits in the bottom of the dorsal furrows in front of the anterior
glabellar furrows. The appendifers on the head in _Calymene_ take the
form of curving projections of shell underneath the glabellar and neck
furrows, and owing to the narrowness of the hypostoma, all these are
visible from the ventral side, even with it in position. This shield
extends back about 0.6 of the length of the cephalon, and to a point
a little behind the second glabellar furrow from the back of the head.
In Doctor Walcott's restoration of _Calymene_ he has represented
all four pairs of biramous appendages as articulating back of the
posterior end of the hypostoma. I think his sections indicate that
the gnathobases of two pairs of these appendages rested alongside or
beneath it, and in particular, the longitudinal sections (1881, pl. 5)
would appear to show that the mouth was some distance in advance of
its posterior end.
_Restoration of Calymene._
(Text fig. 16.)
From what has been said above, it is evident that for a restoration of
the appendages of _Calymene_ considerable dependence must be placed
upon analogy with other trilobites. Nothing is positively known of the
antennules, the exopodites of the cephalon, or any appendages, other
than coxopodites, of the pygidium, but all were probably present. It
is inferred from the slices that the first two pairs of cephalic
appendages were poorly developed, the endopodites short and very
slender, the coxopodites lying parallel to the sides of the hypostoma
and nearly or quite functionless. The gnathites of the last two pairs
of cephalic appendages are large, closely approximated at their inner
ends, and bear small tooth-like spines. The endopodites are probably
somewhat better developed than the anterior ones and more like those
on the thorax.
The coxopodites of the thorax appear to have had nearly cylindrical
endobases which tapered inward. The endopodites were slender, tapering
gradually outward, and probably did not extend beyond the dorsal test.
Small spines were present on the distal end of each segment. Each
exopodite had a long, slender, unsegmented shaft, to which were
attached numerous long, overlapping, flattened setæ. The shaft may
have been angulated near the proximal end, and may have been directed
somewhat forward and outward as in Neolenus, but the evidence on this
point is unsatisfactory. The number of pairs of appendages is that
determined by Walcott from longitudinal sections, namely, four pairs
on the cephalon beside the antennules, thirteen pairs in the thorax,
and nine pairs on the pygidium.
=Calymene= sp. ind.
(pl. 6, figs. 4, 5.)
Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
8, 1881, pl. 6, figs. 5a, b;--Proc. Biol. Soc. Washington, vol. 9,
1894, pl. 1, fig. 10;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8,
fig. 10;--Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2,
2a-d.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12,
1881; pl. 12, figs. 44a, b.
In the United States National Museum there is a thin piece of
limestone, about 3 inches square, which has on its surface eight
jointed objects that have been called legs of trilobites. Two of these
were figured by Walcott (1881, pl. 6, fig. 5). The slab contains
specimens of _Dalmanella_ and _Cryptolithus_, in addition to the
appendages of trilobites, and is said by Doctor Ulrich to have come
from the tipper part of the Point Pleasant formation (Trenton) on the
bank of the Ohio River below Covington, Kentucky.
The specimens are all endopodites of long slender form, similar to
those of _Triarthrus_, but since that genus does not occur in the
Point Pleasant, it is necessary to look upon some other trilobite as
the former possessor of these organs. Both _Isotelus_ and _Calymene_
occur at this horizon, and as the specimens obviously do not belong
to _Isotelus_ or _Cryptolithus_, it is probable that they were
formerly part of a _Calymene_.
All the endopodites are of chitinous material, and the various
specimens show, according to the perfection of their preservation,
from four to six segments. The endopodite as a whole tapers but
slightly outward, and the individual segments are of nearly equal
length. They appear to be but little crushed, and are oval in section,
with a crimped anterior and posterior margin. One or two show a median
longitudinal ridge, such as is seen in some appendages of
_Triarthrus_. Each segment is parallel-sided, with a slight expansion
at the distal end, where the next segment fits into it.
Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154,
pl. 36, figs. 1,2) has recently redescribed these specimens, and
thinks that they do not belong to _Calymene_, nor, indeed, to any
trilobite. He concludes that they were more like what one would expect
in an isopod. Passing over the fact that the oldest isopod now known
is Devonian, the fossils in question seem to me quite trilobite-like.
Walcott says:
The legs are associated with fragments of _Calymene meeki_ but it
is not probable that they belong to that species; if they did, they
are unlike any trilobite leg known to me. The very short coxopodite
and basopodite are unknown in the trilobites of which we have the
legs, as they are fused into one joint forming the long protopodite
in the trilobite. The distal joint is also unlike that of the
trilobite legs known to us.
A great deal of Doctor Walcott's difficulty probably arises from his
homology of the coxopodite of the trilobite with the protopodite of
the higher Crustacea. The coxopodite of the trilobite is not fused
with the basipodite, this latter segment always remaining free.
Indeed, Walcott himself says of _Neolenus_ (1918, p. 128):
Each thoracic leg (endopodite) is formed of a large elongate
proximal joint (protopodite), four strong joints each about 1.5
times as long as wide (basopodite, ischiopodite, meropodite and
carpopodite); two slender elongate joints (propodite and
dactylopodite) and a claw-like, more or less tripartite
termination.
Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it
shows eight segments, I was not able to count more than seven on any
of the specimens themselves. In regard to the terminal segment,
the dactylopodite of the limb shown in his plate 36, figure 2, is
unusually long, and a comparison with other photographs published on
the same plate shows that such long segments are unusual.
Proof that these are appendages of a _Calymene_ is of course wanting,
but there is no particular reason so far to say that they are not.
_Measurements:_ Two of the more complete specimens, each showing six
segments, are each 8 mm. long.
Somewhat similar to the specimens from Covington are the ones
described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being
from the Silurian of Gotland. The figure copied by Walcott (1881, pl.
6, fig. 4) has never been looked upon as entirely satisfactory
evidence of the nature of the specimen, and so far as I know, the
fossil has not been seen by any modern investigator.
=Ceraurus pleurexanthemus= Green.
(pl. 11; text figs. 12, 17-19, 21, 22, 24, 29, 30.)
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
(not _Calymene_), (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.
[Illustration: Fig. 17. Transverse section of _Ceraurus
pleurexanthemus_, showing the relation of the coxopodite to the
appendifer. Traced from a photographic enlargement of the slice.
Specimen 128. × 4/5.]
[Illustration: Fig. 18. Slice of _Ceraurus 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
shows the 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 in
_Calymene_, 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
or _Triarthrus_ that 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 in _Calymene_, 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 to within 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 of _Ceraurus
pleurexanthemus_ at 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; text fig. 19.)
The restoration of the appendages of _Ceraurus pleurexanthemus_ is 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.
[Illustration: Fig. 19. Restoration of a transverse section of the
thorax of _Ceraurus pleurexanthemus_ Green, 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 with
_Triarthrus_.
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 exact form 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 of _Cryptolithus_. 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 of _Acidaspis trentonensis_, obtained from the
same locality and horizon as the specimens of _Triarthrus_ and
_Cryptolithus_, when cleaned from the ventral side shows a number
of poorly preserved endopodites which seem very similar in shape and
position to those of _Triarthrus_. 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
in _Triarthrus_. All that can be seen indicates that _Acidaspis_ had
appendages entirely similar to those of _Triarthrus_, 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.
The Appendages of Cryptolithus.
=Cryptolithus tessellatus= Green.
(pl. 6, fig. 7; pls. 7-9; text figs. 20, 25, 45, 46.)
(See also Part IV.)
Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.
When Professor Beecher wrote his short article on the "Structure
and Appendages of _Trinucleus_" (1895), he had only three specimens
showing appendages. In his later work he cleaned several more, so that
there are now thirteen specimens of _Trinucleus_ = _Cryptolithus_
available 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 a _Trinucleus_ with 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 in _Triarthrus_, 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 in _Triarthrus_ and _Neolenus_. 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._
(Text fig. 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 of _Cryptolithus_ about 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 with _Cryptolithus_ preserved 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 Bohemian _Trinucleoides 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 of _Cryptolithus_ may be explained as an inherited one, since
with the small hypostoma they might have been within the glabella, as
in _Triarthrus_.
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, since in
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.
[Illustration: Fig. 20. _Cryptolithus tessellatus_ Green. 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. There is 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.
Summary on the Ventral Anatomy of Trilobites.
COMPARISON OF APPENDAGES OF DIFFERENT GENERA.
Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_,
_Calymene_, and _Ceraurus_ are now known with some degree of
completeness, those of _Isotelus_ somewhat less fully, and something
at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, 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 of
_Neolenus_ alone, 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.
In _Triarthrus_ and _Neolenus_ the endobases are flattened and taper
somewhat toward the inward end. In _Isotelus_, _Calymene_ and
_Ceraurus_, 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 in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_. In all they are long, slender, and
composed of numerous segments, which are spiniferous in _Neolenus_,
and very probably so in the other genera.
In _Triarthrus_, _Neolenus_, and _Ptychoparia_ they project ahead of
the cephalon, emerging quite close together under the front of the
glabella, one on either side of the median line. In _Cryptolithus_
they turn backward beneath the body, but since only three or four
specimens are known which retain them, it is possible that other
specimens would show that 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 of
_Triarthrus_ are 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 in
_Triarthrus_ is 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 in _Triarthrus_ seem to be
under the inner part of the second pair of glabellar furrows. In
_Cryptolithus_ they 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 in _Neolenus_
it 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 in
_Triarthrus_, _Cryptolithus_, _Neolenus_, _Kootenia_, _Calymene_,
_Ceraurus_, and _Ptychoparia_. In _Triarthrus_, _Neolenus_,
_Acidaspis_, _Ptyclioparia_, and Kootenia, the appendages extend
beyond the margins of the dorsal test. In _Cryptolithus_ and
_Isotelus_ none (other than antennules) does so. In _Isotelus_ and
_Acidaspis_ only the endopodites have been seen. In _Triarthrus_,
_Calymene_, _Ceraurus_, and _Neolenus_ there 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. In _Triarthrus_ the 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. In _Cryptolithus_ the 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. In _Neolenus_ the 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. In _Calymene_ and _Ceraurus_ the endopodites
appear to consist of slender segments which are oval or circular in
section. In _Calymene_ Walcott 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 of _Isotelus_ are 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 of _Acidaspis_, _Kootenia_, and
_Ptychoparia_ are 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 cases function 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. In _Calymene_ and _Ceraurus_, 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. In _Isotelus_, 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 of _Isotelus_, 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 of _Neolenus_ are 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 of _Cryptolithus_ are 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,
_Cryptolithus_ was a mud feeder, the gnathites were probably poorly
developed. Of the gnathites of _Kootenia_, _Ptychoparia_, and
_Acidaspis_ also 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. In _Cryptolithus_ the
exopodites are much the longer. In _Neolenus_, _Calymene_, _Ceraurus_,
_Kootenia_, and _Ptychoparia_, the exopodites are shorter than the
endopodites.
The exopodites in Triarthrus consist 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. In _Cryptolithus_
the 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. In _Calymene_ the
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. In _Ceraurus_ the exopodite seems to have been somewhat
paddle-shaped, expanded at the distal end, and to have had rather
thick, blade-like setæ.
The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one
somewhat of the exites of 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. In _Kootenia_ the
exopodites are like those of _Neolenus_, but with a narrower shaft.
The exopodites of _Ptychoparia_ appear to be very much like those of
Triarthrus, but the shaft is probably not segmented.
The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and
_Acidaspis_ show 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 of _Triarthrus_ bears 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 of
_Cryptolithus_ is 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 of _Ceraurus_, _Calymene_, and _Isotelus_ are 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 of _Isotelus_ are short, the entire six segments of one
being but little longer than the coxopodite of the same appendage. The
segments of the endopodites of _Neolenus_ are mostly short and wide,
and at the distal end of the terminal segment there are three stout
spines. In _Kootenia_ the endopodites are long and very slender. The
endopodites of Ptychoparia are too poorly preserved to show details,
and those of the thorax of _Acidaspis_ likewise 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 in _Triarthrus_ there 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 in
_Neolenus_ this 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 of _Triarthrus_. 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. In _Cryptolithus_ all the pygidial appendages are short
and remain beneath the cover of the dorsal test, while in _Triarthrus_
and _Neolenus_ they 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 be
attached to a narrow rim-like plate which seems to fit in just ahead
of the doublure of the pygidium, or perhaps over it.
In _Ceraurus_, _Calymene_, and _Isotelus_, 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 in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_, there is little reason to think that
they were absent in _Ceraurus_ or _Calymene_, though there is some
question about _Isotelus_.
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.
New _free_ segments 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 an
_Apus_-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 in _Apus_, 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.
The "pygidial antennæ" seem to be as fully developed in _Neolenus_ as
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. In
_Triarthrus_, 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 and _Calymene_, and allowing
the gnathites to play under the hypostoma as Walcott (1912) has shown
that they do in _Marrella_. Finally, when I came to study in greater
detail the slices of _Calymene_ and _Ceraurus_, 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 of
_Ceraurus_. 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
of _Triarthrus_ would 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 saw five 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 in _Marrella_, 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 in
_Calymene_, 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 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 setæ 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 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.
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 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 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 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 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 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 preserved _Dipleura_ in this position is on exhibition in the
Museum of Comparative Zoology, and Mr. Narraway and I have figured a
_Bumastus milleri_ in 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 the
_Dorypyge_ of 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 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 attitude 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 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.
_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 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.