BREATHING ORGANS IN PROSOHRANCUIATA 



157 



111 Cliiton the gills never extend completely round the animal, 

 1)ut are always more or less interrupted at tlie head and anus. 

 They are the true gills, the plumes being serially repeated in the 

 same way as the shell plates. 



Fici. (to. — Patella vulgala L., seen from tlie 

 ventral side : f, foot ; r/./, circlet of gill 

 lamellae; m.e, edge of the mantle; mu, 

 attachment muscle ; si, slits in the same ; 

 *7(, .shell ; v, vessel carrying aerated blood 

 to the heart ; v , vessel carrying blood from 

 the heart ; vc, small accessory vessels. 



Fig. 64. — Patella xmlgata L. , seen from 

 the dorsal side after the removal of the 

 shell and the black pigment covering 

 the integument ; the anterior portion of 

 the mantle is cut away or turned back : 

 a, anus ; hr, br, remains of the true 

 branchiae (ctenidia) ; i, intestine ; k, Ic, 

 kidneys ; l-.a}), their apertui-es on each 

 side of the anus ; /, liver; m, m, mantle; 

 mn, attachment muscles, severed in re- 

 moval of shell ; t, t, tentacles. 



Tn the land Prosobranchiata (Cyclostomatidae, Cyclophoridae, 

 Aciculidae, Helicinidae) which, having exchanged a marine for 

 an aerial life, breathe air instead of water, the branchia has 

 completely disappeared, and breathing is conducted, as in the 

 I'ulmonata, by a lung-cavity. In certain genera of land oper- 

 culates, e.g. Pajpina, Cataulus, Pterocydus, a slight fissure or tube 

 in the last whorl (see Fig. 180, p. 266) serves to introduce air 

 into the shell, which is perhaps otherwise closed to air by the oper- 

 culum. In Aulopoma, which has no tube, the operculum admits 

 free circulation of air. In certain other Cyclostomatidae the apex 

 is truncated, and air can enter there. De Folin closed with wax 

 the aperture of Cycl. elegans, and found that on placing it in a 

 pneumatic machine, the shell gave off air through its whole sur- 

 face. On the other hand, Cylindrella and Stenogyra dccollata. 



