152 



RESPIRATION BY THE SKIN 



lung-cavity, and in CcrWiidca oMusa, which has a puhuunaiy 



Fro. 57. — A, SipJin/iariu gigas. 

 Sowlj., P;uiaiiia, the animal 

 contracted in spirit : gr, 

 sijilional groove on right 

 .side. B, Gadinia i)er}i- 

 riana, Sowh., Cldli, shell 

 only: gr, mark of siphonal 

 groove to riglit of head. 



organisation exactly analogous to that of Cycloj)horus^ this 

 process may l)e regarded as practically completed. 



Eespiration Ijy means of tlie skin, without the development 



of any special organ, is the 

 simplest method of breathing 

 which occurs in the Mollusca. 

 In certain cases, e.g. Elysia, Li- 

 mapontia, and Cenia among the 

 Nudibranchs, and the parasitic 

 JEntoconcha and Entocolax, none 

 of which possess breathing organs 

 of any kind, the whole t)uter 

 surface of the l)ody appears to 

 perform respiratory functions. In 

 others, the dorsal surface is cov- 

 ered with papillae of varied size 

 and number, which communicate 

 with the heart l)y an elaborate 

 system of veins. This is the case 

 with the greater inunber of the 

 Ae olid Ida e (Fig. 58, compare Fig. 

 5, C), but it is curious that when 

 the animal is entirely deprived of 

 these papillae, respiration appears 

 to be carried on without inter- 

 ruption through the skin. 

 In the development of a distinct breathing organ, it would 

 seem as if progress had been made along two definite lines, each 

 ^ Stoliczka, quoted in Journ. de Conch, xviii. p. 452. 



Fig. 58.- 

 coasts 



-Aeolis despecta Johnst., British 

 (After Alder and Hancock. ) 



