ON ONCHNESOMA STEBNSTEUPII. 229 



in the case of Onchnesoma this forms the only circulating 

 fluid. All the organs of the body, the alimentary canal, the 

 nerve-cord, the nephridia, the chief muscles, and the genera- 

 tive organs, are suspended in this fluid, and bathed by it on all 

 sides. The coelomic fluid is kept in constant movement by 

 the protrusion and retraction of the introvert, and by the 

 action of the ciliated peritoneal epithelium which lines the 

 body-wall and covers the internal organs. Thus the corpus- 

 culated coelomic fluid is continually flowing over and circulating 

 around all the organs suspended in it, and there is not much 

 doubt that it acts as a carrier of oxygen to them. 



The problem next arises, where does it eflfect the exchange 

 of gas which constitutes respiration ? This seems capable of 

 two solutions ; the coelomic fluid takes its oxygen either from 

 the corpusculated fluid of the closed vascular space, or through 

 the walls of the alimentary canal. I am inclined to think that 

 the latter alternative is responsible for the chief supply of 

 oxygen to the body. 



The walls of the vascular system are not very thin, and they 

 do not present a very large surface to the coelomic fluid ; and 

 although I think it possible that this fluid acts to a certain 

 extent as a carrier of oxygen, more particularly to the brain, 

 which, except where it is continuous with the epidermis, is 

 surrounded on all sides by it, I still think that the primary 

 function of the closed vascular system is to extend the tentacles 

 by the contraction of its muscular walls forcing fluid into them, 

 and that the primary function of the tentacles is to bring food 

 to the mouth by the action of their cilia. For these reasons I 

 think it, both on morphological and physiological grounds, in* 

 expedient to speak of the tentacles as branchiae. 



The alimentary canal, on the other hand, has very thin 

 walls, and owing to its looped and coiled disposition presents 

 a very large surface to the coelomic fluid. A considerable 

 amount of water must be continually passing through the 

 alimentary canal, since the food of the animal is brought into 

 the oesophagus in a current of water set up by the cilia. This 

 current is set up by the cilia lining the lips and oesophagus. 



