54 



MOLLUSCA. 



mouth and tentacles but is hardly separated off as a distinct head : 

 beneath the mouth there are two large lateral fins which morpho- 

 logically are to be explained as paired parts (epipodia) of the foot 

 (the impaired part is rudimentary), and which, by their wing-like 

 flappings, cause the movement of the animal. The body is either- 

 naked (fig. 526) and -without distinctly separate mantle, or there is 

 a shell of very various shape, into which the body with the fins can 



usually be completely with- 

 drawn, and which may be 

 horny, gelatinous and cartilagi- 

 nous, or calcareous, and is al- 

 most always symmetrical. In 

 the last case (presence of shell) 

 the mantle is usually very com- 

 pletely developed and encloses 

 most of the body to the region 

 of the fins, behind which the 

 slit-like entrance to the mantle 

 cavity is placed. The integu- 

 ment usually contains calcareous 

 concretions, cutaneous glands, 

 and pigment cells, which may 

 give the body a dark brown, 

 sometimes brownish, or even 

 reddish colour. 



The mouth is sometimes sur- 

 rounded by several arm-shaped 

 processes (Clio), or by two 

 beset with suckers 

 (Pneumodermon), the cephalic 

 cones (fig. 526). It leads into 



R 



Fm. 527. Creseis a.cicula from the dorsal side 

 (after Gegenbaur). The hinder part of the body * 

 is omitted. Fl, fins ; 0, mouth ; Oes, oesophagus ; 

 P, median lobe of the foot ; F, tentacle ; Gy, 

 cerebral ganglion ; Mn, mantle nerve ; Ws, cilia- 

 ted shield ; M. stomach; El, blind sac of stomach ; a buCCal Cavity, armed with 

 A, anus ; N, kidney ; Oe, opening of kidney into . 



the mantle cavity ; At, auricle; Ve, ventricle; J aws and toothed radula ; at the 

 Asexual gland; 22, retractor. ' bottom of the mouth the long 



oesophagus begins (fig. 527). The oesophagus leads into a dilated 

 stomach, which is followed by a long, coiled intestine, which is sur- 

 rounded by the liver and bends laterally and forwards. The anus is 

 usually in the mantle cavity on the right side and near the front end. 

 The circulatory organs are reduced to arterial vessels ; the main 

 trunks arise from the spherical ventricle. The veins are replaced by 

 a system of lacunae of the body cavity without special walls, into 



