114 SEA-SIDE STUDIES. 



vibratile cilia, which permits the aeration of all the 

 blood circulating at the surface, but it has also a sys- 

 tem of water-carrying vessels round the margin of its 

 broad foot, or fleshy disc, through which water is car- 

 ried into the cavities of the bodv, and there aerates the 

 blood, — at least I assume this latter j)art of the descrip- 

 tion to be true of all Eolids, as it is of the Doris, 

 although I only observed the vessels in one species, not 

 having thought of seeking them until my last animal 

 was under the scalpel. 



Having made thus much clear to myself, I found that 

 I was not so heretical as I fancied, but that, for the 

 main facts on which my argument rested, the authority 

 of even Alder and Hancock could be claimed. These 

 admirable authors distinctly say, " the respiratory func- 

 tion appears to be partially specialised in the dorsal 

 papillse, which, usually exposing a large surface, are 

 covered with strong and vigorous vibratile cilia. [Not 

 so strong as the cilia of the foot, however.] But as the 

 blood in its return to the heart appears to pass almost 

 entirely through the skm, which is thin and delicate, 

 and also covered with cilia, there can be little doubt 

 that the luliole 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 giU-structure 

 in the papiUae, to perceive at once the impropriety of 

 designating those organs as branchial, and of including 

 the Eolis among Nudibranchiates. Por although the 

 papillae expose a large surface to the air, they only do 

 so in common with the rest of the skin ; mere extent of 



