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Professor Brooks has called attention to the fact that the higher a group stands in development, the longer its ancestors have maintained a swimming life. Thus we have noticed that the sponges were the first to settle; then a little later the mass of the coelenterates followed their example. But the etenophora, the nearest relatives of bilateral animals, have remained free swimming.
COELENTERATES. This subkingdom includes two classes of interest to the geologist, the HYDROZOA, such as the fresh-water hydra and the jellyfish, and the CORALS. Both classes existed in the Cambrian. The Hydrozoa were represented not only by jellyfish but also by the GRAPTOLITE, which takes its name from a fancied resemblance of some of its forms to a quill pen.
All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways. We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced.
With more or less modification in detail, the embryo has been observed to pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief that all animals of higher organisation than the Protozoa, agree in the general character of the early stages of their individual evolution.
I think we may go farther, although in this latter consideration we may very possibly be mistaken. We have already seen that the progress made by any animal may be measured more or less accurately by the length of time during which its ancestors maintained a swimming life. The ancestors of the coelenterates settled to the bottom first.
Each of these early embryological stages is represented by living animals, the undivided cell by the PROTOZOA, the blastosphere by some rare forms, and the gastrula in the essential structure of the COELENTERATES, the subkingdom to which the fresh-water hydra and the corals belong.
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