EOLIS NOT A NUDIBRANCH. 109 



and also covered with cilia, there can be little doubt tliat the 

 ichole surface of the body assists in aeration." After this, 

 it seems to me that these authors need only reflect a moment 

 on the absence of gill-structure in the papillge, to perceive 

 at once tlie impropriety of designating those organs as 

 branchial, and of including the Eolis among Nudibranchiates. 

 For although the papillae expose a large surface to the air, 

 they only do so in common with the rest of the integument ; 

 mere extent of surface does not constitute a gill ; nor will a 

 merely respiratory surface constitute one. The frog, for 

 example, respires by the skin, as well as by the lungs, but 

 we do not call its skin a lung, because a lung has a specific 

 structure, widely dift'ering from the structure of the sldn. 



In their magnificent monograph, Messrs Alder and Han- 

 cock say : " The order Nudibranchiata is restricted to those 

 animals bearing the character assigned to it by Cuvier ; 

 namely, the possession of distinct external and uncovered 

 gills." But we have just seen that it is precisely tlii.s 

 character which is wanting in the Eolids ; nor do I under- 

 stand how these learned authors come to include Eolids 

 among the Nudibranchiates, unless they also proposed to 

 annihilate the very terms of their definition. If the Eolids 

 have not distinct external and uncovered gills — no gills at 

 all, in fact — they cannot lawfully claim a place among the 

 Naked-gilled Molluscs. 



The reader will understand that at the coast, where these 

 dissections were made, I had but a scanty supply of boolcs 

 to aid me, and that on returning home a diligent search was 

 made, with a view of ascertaining what had been already 



