If we take the genus arca, for instance, as a typical mollusk, it is easy to follow the segments of which it is composed. Each half of this bivalve has its own organization, as complete as in any segment of any annulosa. Each has its own heart, its own nerve-ganglia and branches, its own aërating apparatus and systemic circulation, such as it is. You may even trace, still, the crossing of muscles and nerve-fibres, at the back or hinge, exactly as between the two articulate segments from which they sprang.
Here it may be remarked, again, how tenacious Nature is of her plans. This mysterious crossing and anastomosing of nerve-fibres, so unaccountable in the brain of the higher animals, and of such important consequences even in man himself, had its origin in the primal union of two annulose segments.
Having overcome the principal difficulty by noticing the change of axis, nothing more remains but to pursue this bisegmental arrangement to its full development in the grand class of mollusks. Possibly not one of existing species had any part in the chain of development. But this is of small interest at present, since we are looking for the method, the steps, the finger-boards of the road traveled, and care net now to count the milestones.
The transition from the highest mollusca to vertebrata, as already remarked, is plain enough. Mollusca already have the internal structure of vertebrata—the same digestive system—similar nerves—and identical circulatory machinery. The highest mollusks have as much brain as the lowest fishes, and decidedly more than the famous amphioxus—a creature which just now is an obstruction instead of a help in the establishment of a sound biological theory of development; and this for the reason that it is leading us away from the true relations of these orders, and helping to keep up the old misconception of the nature, origin, and importance of a jointed spinal column. Unfortunately, it is this want of vertebræ, of backbone in mollusks, that prevents us from seeing the near alliance of cephalopods to vertebrates. Had the latter been supplied with the more appropriate and distinctive title Cerebrata, these highest mollusks might better have claimed admission to the class than many species now found in its ranks. For, besides the homologues already mentioned, do but notice the optic and auditory apparatus of sepia, for instance. These are perfectly identical with the eyes and ears of vertebrates. Notice, again, the organization of the mouth. Only vertebrates have such. In annulosa the jaws have lateral motion, and are modifications of the feet. One other consideration must be mentioned—the size of these animals. Only vertebrata and mollusca seem to have unlimited powers of increase. They only have been distinguished for the magnitude of individuals. Among the latter especially are to be noted the cephalopods for furnishing giants. Did space permit, a thousand homologues might be pointed out.
