MOLLUSKS IN GENERAL 229 



and the outspread tentacles of the largest octopus possess a span of 30 

 feet. Among living types the shelled ones are relatively small, but some 

 fossil forms with shells reached a large size. The shells of the straight- 

 shelled Silurian types exhibit a length of 15 to 18 feet, and those of the 

 coiled ammonoids of the Jurassic, which resembled the living pearly 

 nautilus, a diameter of from 4 to 6 feet (Sec. 586 and Fig, 372). 



265. Metabolism, — The food of mollusks is exceedingly varied. The 

 pelecypods take any minute particles of organic matter found in the 

 water, while the snails and their allies scrape the epidermis from living 

 or dead plants or gather algae from the surfaces on which they grow. 

 On the other hand, the cephalopods, powerful and active forms as they 

 are, are carnivorous and will feed upon any animal which they can over- 

 come. Squids destroy a great many fish, and an octopus will undertake 

 to capture anything which it can grasp with its tentacles. One large 

 marine gastropod, Sycotypus, lives in shallow water and feeds on other 

 mollusks, while another, known as the oyster drill, Urosalpinx, feeds 

 on oysters and other bivalves, boring a hole through the shell of the victim 

 with its radula and eating out the soft parts. 



266. Behavior. — The behavior of the most simple types of mollusks 

 is correspondingly simple, but that of the cephalopods is very complex. 

 One striking fact about mollusks is that in the case of the lowest class, 

 the Amphineura, which is wormlike, there is a nervous system similar in 

 some respects to that of the fiatworms. In other classes there are numer- 

 ous scattered ganglia. These in the cephalopods are grouped together 

 in the head. The close proximity of these ganglia offers an opportunity 

 for quick communication between them and effective coordination, pro- 

 duces a real brain, and has resulted in developing a very efficient 

 organism, to which some zoologists have attributed the possession of 

 intelligence. 



The sense which most mollusks depend upon is that of smell. With 

 few exceptions they seem to be sensitive to light, and it has already been 

 shown that the cephalopods have excellent vision. 



267. Reproduction and Regeneration. — Asexual reproduction never 

 occurs in MoUusca, but the animals may be either monecious or diecious. 

 Though some of them produce only a few eggs, others produce large 

 numbers. It is stated, for example, that 9,000,000 eggs may be laid by 

 a single oyster. Some snails, and also some small fresh-water bivalve 

 mollusks, are viviparous. 



Molluscan eggs are holoblastic but undergo unequal cleavage. Devel- 

 opment typically includes a trochophore stage but this is not represented 

 in the development of land and fresh-water mollusks. The cephalopods 

 have a larva which develops within the egg. The trochophore larva 

 (Fig. 141) is top-shaped with a ring of ciHa about the margin of the 

 expanded upper portion and with an eyespot at the apex of the body. 



