108 SEA-SIDE STUDIES. 



property of A ssimilation, and the special function of Diges- 

 tion. The Eolis we are considering must have its blood 

 aerated ; but the means by which it is aerated do not come 

 under the term " breathing." In many of the lower animals 

 aeration is performed entirely by the surface, the air or 

 water directly bathing the delicate tissues, and bringing to 

 them the necessary supply of oxygen, without the inteiTen- 

 tion of any special apparatus ; just as food is brought to 

 their tissues without the preparatory labour of arduous 

 digestion. The Eolis has not only a delicate surface, covered 

 with vibratile cilia, which permits the aeration of all the 

 blood circulating at the surface, but it has also a system of 

 aquiferous pores round the margin of its broad foot, or fleshy 

 disc, through which water is carried into the peritoneal 

 cavities and there aerates the blood, — at least I assume this 

 latter part of the description to be true of all Eolids, as it is 

 of the Doris, although I only observed the pores 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, I could claim the authority 

 of even Alder and Hancock. These admirable authors dis- 

 tinctly say, " the respiratory function appears to be partially 

 specialised in the dorsal papill.ne, 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 entkely through the skin, which is thin and delicate. 



