MOLLUSKS 91 



the eye of the squids and cuttlefishes has shown them 

 to be remarkably complex and in many respects to be 

 constructed upon much the same plan as those of the 

 vertebrates. As to the other senses not so much is known, 

 but undoubtedly many species of cephalopods are possessed 

 of a shrewdness and cunning not shared by any other 

 invertebrates, save some of the insects and spiders, and are 

 vastly more highly organized than their molluscan rela- 

 tives. 



91. How species originate. We have now examined a 

 considerable portion of the animal kingdom, tracing its 

 members from their simplest beginnings as single cells, 

 through the formation of colonial types, and up through 

 the sponges, ccelenterates, worms, and mollusks. It is im- 

 portant once more to note that they all perform the func- 

 tions concerned in nutrition and reproduction, and only 

 these. The differences which exist are those of structure. 

 The Hydra and the clam, for example, perform the same 

 duties, but their bodily apparatus differs widely, and the 

 completeness and perfection of the work varies accord- 

 ingly. The more the work to be performed by an organ- 

 ism is divided up among especially adapted organs, so that 

 each of the latter has, as far as possible, only one thing to 

 do, the higher is the organism. 



As stated earlier in the account, it is believed that the 

 more complex animals arose from the simpler ; that if we 

 could trace the history of any of the great groups back 

 toward their first beginnings, we would find them all to 

 have originated from one ancestral form, that in turn owes 

 its descent from yet simpler forms. 



Let us see something of how this has come about. We 

 all know that vast numbers of young are born into this 

 world which never come to maturity. It is said that if all 

 the young of the codfish were to live their allotted time, 

 it would be less than fifteen years before the sea would 

 become literally packed with them. Numerous enemies, 



