THE CHAIX OF SPECIES. 573 



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 aerating apparatus and systemic circulation, such as 

 it is. You may even trace, still, the crossing of muscles and nerve- 

 tibres, at the back or hinge, exactly as between the two articulate seg- 

 ments 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 impor- 

 tant 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 arrange- 

 ment to its full development in the grand class of mollusks. Possi- 

 bly not one of existing species had any part in the chain of develop- 

 ment. 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 not now to count the milestones. 



The transition from the highest moUusca to vertebrata, as already 

 remarked, is plain enough. Mollusca already have the internal struct- 

 ure 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 develop- 

 ment ; 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 misconcep- 

 tion of the nature, origin, and importance of a jointed spinal column. 

 Unfortunately, it is this want of vertebrse, of backbone in mollusks, 

 that prevents us from seeing the near alliance of cephalopods to verte- 

 brates. Had the latter been supplied with the more appropriate and 

 distinctive title Cerehrata^ 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 no- 

 tice 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 mag- 

 nitude 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. 



