EOLIS NOT A NUDIB RANCH. 115 



surface does not constitute a gill ; nor will a merely 

 resjDiratory surface constitute one. The frog, for ex- 

 ample, 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 differing from the structure 

 of the skin. 



In their monograph, IMessrs Alder and Hancock say : 

 " The order Nudibranchiata is restricted to those ani- 

 mals bearing the character assigned to it by Cuvier ; 

 namely, the j^ossessioji of distinct external and uncov- 

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

 this character which is wanting in the Eolids ; nor do I 

 understand how these learned authors come to include 

 Eolids among the Nudibranchiates, unless they also 

 proposed to annihilate the very terms of their defini- 

 tion. If the Eolids have not distinct external and 

 uncovered gills — if they have no gills at all, in fact — 

 they cannot lawfully claim a place among the Naked- 

 giUed Molluscs. 



The reader will understand that at the coast, where 

 these dissections were made, I had but a scanty supply 

 of books to aid me, and that on returning home a dili- 

 gent search was made, with a view of ascertaining 

 what had been already noted on this point. Owen and 

 Siebold assured me that I was right in denying the 

 Eolids anything properly to be styled a gill. The 

 former says : " In certain small shell-less marine gen- 

 era — e. g. Rhodoj)e, Tergipes, Eolidina — no distinct 

 respiratory organs have been detected ; these form the 

 order Apneusta!' Professor Owen enters into no de- 



