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 shell was secreted on the dorsal surface, covering the whole body. Such a shell need not have been segmented, and, in fact, the stiffer the shell, 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 Pædeumias. (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, Acidaspidæ particularly, have backward projecting spines on the posterior margin of the cephalon, which suggest the possibility of the production of such a shield, 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 antennæ were produced, while the anomomeristic character of the trilobite was retained. From similar opisthoparian ancestors there were, however, derived primitive Malacostraca retaining biramous antennæ, 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.

Fig. 36.—Naraoia compacta Walcott. An outline of the test, after Walcott. Natural size. Fig. 37.—Pagetia clytia Walcott. An eodiscid with compound eyes. After Walcott. × 5.
Fig. 38.—Asaphiscus wheeleri Meek. A representative trilobite of the Middle Cambrian of the Pacific province. After Meek. × 1/2. Fig. 39.—Pædeumias robsonensis Burling. Restored from a photograph published by Burling. × 1/4. Fig. 40.—Robergia sp. Restored from fragments found in the Athens shale (Lower Middle Ordovician), at Saltville, Va. Natural size.

It is customary to speak of the Crustacea and Trilobita as having had a common ancestry, rather than the former being in direct line of descent from the latter, but when it can be shown that the higher Crustacea are all derivable from the Trilobita, and that they possess no characteristics which need have been inherited from any other source than that group, it seems needless to postulate the evolution of the same organs along two lines of development.

I can not go into the question of which are more primitive, sessile or stalked eyes, but considering the various types found among the trilobites, one can but feel that the stalked eyes are not the most simple. While no trilobite had movable stalked eyes, it is possible to homologize free cheeks with such structures. They always bear the visual surface, and, in certain trilobites (Cyclopyge), the entire cheek is broken up into lenses. Since a free cheek is a separate entity, it is conceivable that it might lie modified into a movable organ.

EVOLUTION OF THE MEROSTOMATA.

It has been pointed out above that the Limulava (Sidneyia, Amiella, Emeraldella) have certain characteristics in common with the trilobites on the one hand and the Eurypterida on the other. These relationships have been emphasized by Walcott, who derives the Eurypterida through the Limulava and the Aglaspina from the Trilobita. The Limulava may be derived from the Trilobita, but indicate a line somewhat different from that of the remainder of the Crustacea. In this line the second cephalic appendages do not become antennæ. and the axial lobe seems to broaden out, so that the pleural lobes become an integral part of the body. As in the modern Crustacea, the pygidium is reduced to the anal plate, and this grows out into a spine-like telson.