ON ONCHNESOMA STEENSTRUPII. 245 
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 ceelomic 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 effect the exchange 
of gas which constitutes respiration? This seems capable of 
two solutions: the celomic 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 ccelomic 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 branchiz. 
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 ccelomic fluid. A considerable 
amount of water must be continually passing through the 
alimentary canal, since the food of the animal is brought into 
the cesophagus in a current of water set up by the cilia. This 
current is set up by the cilia lining the lips and cesophagus, 
