Introduction to Animal Morphology. 281 



■cells, which are not acted on by muscular fibres (like 

 the chromatophores of Pteropoda). Cnidae, like those 

 of Turbellaria, exist in clusters on the ends of the dorsal 

 papillae of .^olis and its allies. Bristles arising in 

 follicles adorn the sides of some Chitons. The sub- 

 cutaneous muscular lamina is not easily separable 

 from the dermis, and usually consists of external 

 longitudinal, and internal circular bundles of flat, 

 anastomosing, unstriped fibres, not in separate layers 

 (they are otherwise arranged in -^olidae). These 

 structures make up the body wall, of which two parts 

 are generally differentiated — ist, the post-cephalic 

 neural surface or foot ; 2nd, the haemal surface, which 

 usually forms a mantle. The foot is usually flat, sole- 

 shaped for crawling, and is then rarely divided.* In 

 Heteropoda it is differentiated into three parts — pro-, 

 meso-, and meta-podium — and forms a vertical swim- 

 ming organ or fin, moving as a screw propeller. The 

 upper and hinder edge of the foot may be prolonged 

 into lateral lobes or epipodia, which may be elongated 

 and wavy (Haliotis, Aplysia, Turbo), large and lobed 

 (Rissoa), formed into lateral swimming fins (Gaste- 

 ropteron), or as a pseudo-mantle (Doris). In Stylo- 

 cheilus the metapodium is very long. The foot is a 

 thickening of the body wall with well-developed 

 longitudinal muscular fibres, often with large mucous 

 glands. t The upper surface of the metapodium bears 

 often a differentiated area, secreting a shelly structure, 



* In Harpa, Ancillaria, Voluta, Oliva, the propodium is indicated by a 

 transverse groove. In Strombus the pro- and meso-podium are marked 

 out by grooves. A longitudinal groove divides it in Phasianella, and the 

 two sides can move independently. Transverse grooves exist in Pedipes 

 and Auricula brunnea. 



t In Janthina these are large, and secrete the egg-raft. 



