296 MANUAL OF ZOOLOGY sect. 



such as the limpets, ear-shells, cowries, tritons, whelks, and 

 cones. The latter includes the water-breathing sea-hares 

 and nudibranchs and the air-breathing snails and slugs. 

 The chief general points of distinction between the two 

 groups are that in the Streptoneura the visceral nerve-cords 

 are twisted into a figure 8, and the sexes are separate, 

 while in the Euthyneura the twisting of the nerve-cords is 

 absent and the sexes are united in the same individual. 



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 three 

 thousand fathoms — in the ocean. Many forms, however, 

 are inhabitants of fresh water, while many Pulmonata are 

 terrestrial, and occur even towards the summits of the 

 highest mountains. 



4. THE CEPHALOPODA 



The class Cephalopoda, including the cuttle-fishes, squids, 

 Octopi, argonauts, and Nautili, is the highest of the Mollusca, 

 its members being very much more active and powerful 

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

 more highly endowed as regard their higher senses. The 



