152 



RESPIRATION BY THE SKIN 



CHAP. 



only a lung-cavitj-, and in Cerithidea obtusa, which has a pul- 



FiG. 57. — A, Siphonaria gigas 

 Sowb., Pauama, the animal 

 contracted in spirit: gr, 

 siphoual groove on right 

 side. B, Gadbiia pez-w- 

 viana, Sowb., Chili, shell 

 only: grr, mark of siphonal 

 groove to right of head. 



monary organisation exactly analogous to that of Cyclophorus} 

 this process may be regarded as practically completed. 



Respiration by means of the 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 j)arasitic 

 Entoconclia and Entoeolax^ none 

 of which possess breathing organs 



11 %W of any kind, the whole outer 



\ >^ 4^^S^"'^4\? surface of the body 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 by an elaborate 

 system of veins. This is the case 

 with the greater number of the 

 Aeolidiclae (Fig. 58, compare Fig. 

 •'), C), but it is curious that when 

 the animal is entirel}^ deprived of 

 „ ^„ . ,. , -r , ^ . these papillae, respiration appears 



Fig. 58. — -leo/isdespec^aJohnst., British ^ ^ ^ ^. 



coasts. (After Alder and Hancock.) 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 

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



