184 



INVERTEBRATE ANIMALS. 



water-breathing Gasteropods are divided into two sections 

 differing a good deal from the typical forms of the class in 

 many respects. 



As examples of the first of these may be taken the sea- 

 slugs and sea-lemons {Nudibranchiata), specimens of which 

 may at any time be found creeping about on sea-weeds, or ai> 

 tached to the under surface of stones at low water. These 

 slug-like animals (Fig. 85) are wholly destitute of a shell when 



fully grown, but possess an em- 

 bryonic shell when young. When 

 there are any distinct respiratory 

 organs, these are in the form of 

 gills, placed, without any protec- 

 tion, upon 'the back or sides of 

 the body. The head is furnished 

 with tentacles, which do not ap- 

 pear to be used as organs of touch, 

 but are more probably connected with the sense of smell; and 

 behind the tentacles are generally two eyes. The nervous 

 system is extremely well developed, and would lead to the be- 

 lief that the Sea-slugs are among the highest of the Gastero- 

 poda. Locomotion is effected, as in the true Slugs, by creep- 

 ing about on the flattened foot. 



The last remaining group of the " branchiate " Gasteropods 

 is that of the Seteropoda (Fig. 86), comprising a number of 

 curious forms which are found swimming at the surface of the 



Fio. 85.— Nudibranchiata. Doris John- 

 stoni, one of the Sea-lemons. 



Fig. 86. — Ret&ropoda. Ca/rvmxHa cymbium; p Proboscis and month; 

 < Tentacles; 17 GUIs; « Shell; /Foot; d Disk (after Woodward). 



open sea, instead of creeping about at the bottom of the sea. 

 In order to adapt them for this mode of life, the foot, instead 

 of forming a creeping disk, is modified to form a compressed 

 fin (/■). The Heteropoda are to be regarded as the most 



