Part 24 (1/2)
The Branchiopoda were much more abundant and much more highly diversified in Cambrian times than were the Leptostraca, and, therefore, are probably older. Some of the Cambrian branchiopods were without a carapace, and some were sessile-eyed. These were more trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had developed a bivalved carapace, though not so large a one as that of the primitive Leptostraca. The present indications are, therefore, that the Branchiopoda are really older than the Leptostraca, and also that the latter were derived from them. It seems very generally agreed that the Malacostraca are descended from the Leptostraca, and the fossils of the Pennsylvanian supply a number of links in the chain of descent. Thus, _Pygocephalus cooperi_, with its brood pouches, is believed by Calman (1909, p. 181) to stand at the base of the Peracaridan series of orders, and _Uronectes_, _Palaeocaris_, and the like are Palaeozoic representatives of the Syncarida. Others of the Pennsylvanian species appear to tend in the direction of the Stomatopoda, whose true representatives have been found in the Jura.s.sic. The Isopoda seem to be the only group of Malacostraca not readily connected up with the Leptostraca. Their depressed form, their sessile-eyes, and their antiquity all combine to indicate a separate origin for the group, and it has already been pointed out how readily they can be derived directly from the trilobite.
While the Copepoda seem to have been derived directly from the Hypoparia, the remainder of the Crustacea apparently branched off after the compound eyes became fully developed, unless, as seems entirely possible, compound eyes have been developed independently in various groups. Most Crustacea were derived from crawling trilobites (Lower Cambrian or pre-Cambrian Opisthoparia), for they lost the large pygidium, and also the major part of the pleural lobes. In all Crustacea, too, other than the Copepoda and Ostracoda, there is a tendency to lose the exopodites of the antennae.
These modifications, which produced a considerable difference in the general appearance of the animal, are easily understood. As has been shown in previous pages, the trilobites themselves exhibit the degenerative effect on the anterior appendages of the backward movement of the mouth, and the transformation of a biramous appendage with an endobase into a uniramous antenna is a simple result of such a process. The feeding habits of the trilobites were peculiar and specialized, and it is natural that some members of the group should have broken away from them. In any progressive mode of browsing the hypostoma was a hindrance, so was soon gotten rid of, and the endobases not grouped around the mouth likewise became functionless.
The chief factor in the development of the higher Crustacea seems to have been the pinching claw, by means of which food could be conveyed to the mouth. It had the same place in crustacean development that the opposable thumb is believed to have had in that of man.
An intermediate stage between the Trilobita and the higher Crustacea is at last exhibited to us by the wonderful, but unfortunately rather specialized _Marrella_, already described. It retains the hypostoma and the undifferentiated biramous appendages of the trilobite, but has uniramous antennae, there are no endobases on the c.o.xopodites of the thoracic appendages, the pygidium is reduced to a single segment, and the lateral lobes of the thorax are also much reduced. _Marrella_ is far from being the simplest of its group, but is the only example which survived even down to Middle Cambrian times of what was probably once an important series of species transitional between the trilobites and the higher Crustacea.
In this theory of the origin of the Crustacea from the Trilobita, the nauplius becomes explicable and points very definitely to the ancestor. According to Calman (1909, p. 23):
The typical nauplius has an oval unsegmented body and three pairs of limbs, corresponding to the antennules, antennas, and mandibles of the adult. The antennules are uniramous, the others biramous, and all three pairs are used in swimming. The antennae may have a spiniform or hooked masticatory process at the base, and share with the mandibles which have a similar process, the function of seizing and masticating the food. The mouth is overhung by a large labrum or upper lip and the integument of the dorsal surface of the body forms a more or less definite dorsal s.h.i.+eld. The paired eyes are as yet wanting, but the median eye is large and conspicuous.
The large labrum or hypostoma, the biramous character of the appendages, especially of the antennae, the functional gnathobases on the second and third appendages, and the oval unsegmented s.h.i.+eld are all characteristics of the trilobites, and it is interesting to note that all nauplii have the free-swimming habit.
The effect of inheritance and modification through millions of generations is also shown in the nauplius, but rather less than would be expected. The most important modification is the temporary suppression of the posterior pairs of appendages of the head, so that they are generally developed later than the thoracic limbs. The median or nauplius eye has not yet been found in trilobites, and if it is, as it appears to be, a specialized eye, it has probably arisen since the later Crustacea pa.s.sed the trilobite stage in their phylogeny.
The oldest Crustacea, other than trilobites, so far known are the Branchiopoda and Phyllocarida described by Walcott and discussed above. It is important to note that while the former have already achieved such modified characteristics that they have been referred to modern orders, they retain the trilobite-like limbs and some of them still have well developed pleural lobes.
Calman (1909, p. 101) says of the Copepoda:
On the hypothesis that the nauplius represents the ancestral type of the Crustacea, the Eucopepoda would be regarded as the most primitive existing members of the cla.s.s, retaining as they do, naupliar characters in the form of the first three pairs of appendages and in the absence of paired eyes and of a sh.e.l.l-fold.
As already indicated, however, it is much more probable that they are to be regarded as a specialized and in some respects degenerate group which, while retaining, in some cases, a very primitive structure of the cephalic appendages, has diverged from the ancestral stock in the reduction of the number of somites, the loss of the paired eyes and the sh.e.l.l-fold, and the simplified form of the trunk-limbs.
If the Eucopepoda be viewed in the light of the theory of descent here suggested, it is at once seen that while they are modified and specialized, they more nearly approximate the hypothetical ancestor than any other living Crustacea. Compound eyes are absent, and it can not be proved that they were ever present, although Grobben is said to have observed rudiments of them in the development of _Cala.n.u.s_. The ”simplified limbs” are the simple limbs of the trilobite, somewhat modified. The absence of the sh.e.l.l-fold and carapace is certainly a primitive characteristic. Add to this the direct development of the small number of segments, and the infolded pleural lobes, and it must be admitted that the group presents more trilobite-like characteristics than any other. It seems very likely that the primitive features were retained because of the pelagic habitat of a large part of the group.
Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pl.) has recently outlined a possible method of derivation of the acorn barnacles from the phyllocarids. Starting from a recent _Bala.n.u.s_ with rostrum and carina separated by two pairs of lateralia, he traces back through _Calophragmus_ with three pairs of lateralia to _Protobala.n.u.s_ of the Devonian with five pairs. Still older is the newly discovered _Eobala.n.u.s_ of the upper Ordovician, which also has five pairs of lateralia but the middle pair is reversed, so that when the lateralia of each side are fitted together, they form a pair of s.h.i.+elds like those of _Rhinocaris_, separated by the rostrum and carina, which are supposed to be h.o.m.ologous with the rostrum and dorsal plate of the Phyllocarida. Ruedemann suggests that the ancestral phyllocarid attached itself by the head, dorsal side downward, and the lateralia were developed from the two valves of the carapace during its upward migration, to protect the ventral side exposed in the new position.
This theory is very ingenious, but has not been fully published at the time of writing, and it seems very doubtful if it can be sustained.
_Summary._
The salient points in the preceding discussion should be disentangled from their setting and put forward in a brief summary.
It is argued that the ancestral arthropod was a short and wide pelagic animal of few segments, which so far changed its habits as to settle upon a substratum. As a result of change in feeding habits, appendages were developed, and, due perhaps to physiological change induced by changed food, a sh.e.l.l was secreted on the dorsal surface, covering the whole body. Such a sh.e.l.l need not have been segmented, and, in fact, the stiffer the sh.e.l.l, the more reason for development of the appendages. Activity as a swimming and crawling animal tended to break up the dorsal test into segments corresponding to those of the soft parts, and, by adaptation, a floating animal became a crawling one, with consequent change from a form like that of _Naraoia_ to one like _Paedeumias_. (See figs. 36-40.) A continuation of this line of development by breaking up and loss of the dorsal test led through forms similar to _Marrella_ to the Branchiopoda of the Cambrian, in which not only is there great reduction in the test, but also loss of appendages. The origin of the carapace is still obscure, but Bernard (1892, p. 214, fig. 48) has already pointed out that some trilobites, Acidaspidae particularly, have backward projecting spines on the posterior margin of the cephalon, which suggest the possibility of the production of such a s.h.i.+eld, and in _Marrella_ such spines are so extravagantly developed as almost to confirm the probability of such origin. In this line of development two pairs of tactile antennae were produced, while the anomomeristic character of the trilobite was retained. From similar opisthoparian ancestors there were, however, derived primitive Malacostraca retaining biramous antennae, but with a carapace and reduced pleural lobes and pygidium. From this offshoot were probably derived the Ostracoda, the Cirripedia, and the various orders of the Malacostraca, with the possible exception of the Isopoda. I have suggested independent origins of the Copepoda and Isopoda, but realize the weighty arguments which can be adduced against such an interpretation.
[Ill.u.s.tration: Fig. 36.--_Naraoia compacta_ Walcott. An outline of the test, after Walcott. Natural size.]
[Ill.u.s.tration: Fig. 37.--_Pagetia clytia_ Walcott. An eodiscid with compound eyes. After Walcott. 5.]
[Ill.u.s.tration: Fig. 38.--_Asaphiscus wheeleri_ Meek. A representative trilobite of the Middle Cambrian of the Pacific province. After Meek.
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[Ill.u.s.tration: Fig. 39.--_Paedeumias robsonensis_ Burling. Restored from a photograph published by Burling. 1/4.]
[Ill.u.s.tration: Fig. 40.--_Robergia_ sp. Restored from fragments found in the Athens shale (Lower Middle Ordovician), at Saltville, Va.
Natural size.]