296 MANUAL OF ZOOLOGY SECT. 



cords are twisted into a figure of 8, and the sexes are separ- 

 ate, while in the Euthyneura the twisting of the nerve-cords 

 does not occur, and the sexes are united. 



Only a few aberrant families of Gastropoda are parasites. 

 Most are aquatic, all the most primitive forms being in- 

 habitants of the sea. Of the marine families the majority 

 move by creeping over the sea-bottom, some burrowing in 

 mud or sand, some in solid rock ; some are able to float in 

 a reversed position, adhering to frothy mucus secreted by 

 the glands of the foot ; certain exceptional forms such as 

 Vermetus are fixed in the adult condition by the substance 

 of the shell. A few families the Heteropoda and the 

 Pteropoda are specially modified for a pelagic mode of 

 existence, and swim through the water by flapping move- 

 ments of the lobes of the foot, which act as fins. Gastro- 

 pods are found at considerable depths up to nearly 3000 

 fathoms in the ocean. Many forms, however, are in- 

 habitants of fresh water, while many Pulmonata are 

 terrestrial, and occur even towards the summits of the 

 highest mountains. 



4. THE CEPHALOPODA 



The class Cephalopoda includes the highest of the 

 Mollusca, viz., the Cuttle-fishes, Squids, Octopi, Argonauts, 

 and Nautili. These are very much more active and power- 

 ful in their movements than the rest of the Mollusca, and 

 much more highly endowed as regard their higher 

 senses. The body (Figs. 170, 171, 173, 175) is bilaterally 

 symmetrical. The foot, instead of extending along the 

 ventral surface of the body in the region behind the mouth, 

 as it does in Pelecypoda and Gastropoda, occupies a more 

 anterior position, and surrounds the mouth. A distinct head 



