222 GENERAL ZOOLOGY 



sea- weed (C). She then leaves them to develop without 

 further care. 



Squids are commonly eaten for food in Italy and are com- 

 ing to be used in other countries. They are also much sought 

 for bait by fishermen. Though some squids attain a total 

 length of forty feet, they are as a rule very shy, and their 

 reputed attacks on men are probably fictitious. 



GENERAL REMARKS ON MOLLUSCS 



The Mollusca are remarkable among other phyla of 

 bilaterally symmetrical animals because they are complex 

 in structure but strictly non-met americ. Though the tro- 

 chophore-like larva (page 190) indicates possible affinities 

 with the Annelida, Echinodermata, Rotifera, etc., the 

 structure of adult molluscs is wholly unlike that of other 

 animals. The Mollusca therefore have no close relation- 

 ship with any other phylum, and have developed along 

 original lines. 



The most striking characteristics of the phylum as a whole 

 are the foot, the shell, the mantle, and the simple nervous 

 system, but all these structures differ greatly in different 

 classes. The foot is flat in gastropods, hatchet-shaped in 

 lamellibranchs, and divided into eight, ten, or more arms in 

 cephalopods. The shell is univalve in gastropods, bivalve 

 in lamellibranchs, and usually rudimentary or absent in 

 cephalopods.' Squids and devil-fishes have a sac-like 

 mantle which encloses most of the body ; clams usually have 

 the mantle divided into two great lobes, completely cover- 

 ing the soft body; but in snails the greater part of the body 

 may be extended outside the mantle cavity. 



Except in cephalopods the nervous system is as a rule 

 poorly developed and its structure is rather similar in all the 

 classes. It consists of three or four pairs of ganglia and a 

 few connective nerves. Molluscs show in a striking way 

 that the degree of specialization in nervous structures is 

 associated with activity. Most of the slow, sluggish clams 



