268 A MONOGRAni OF THE AUSTRALIAN APHRODITEA, 



of ** brancliIaB." Here the integument is usually rather thinner 

 than in other parts, and is often free from pigment ; in Aphrodita 

 and Iphione it is perfectly transparent. These tubercles are 

 occupied, as already described, by the terminal portions of the 

 dorsal caeca, which curve round above the dorsal muscles, so as to 

 lie in close contact with the integument. The peculiarity of this 

 arrangement was remarked upon long ago by Williams.*' He 

 remarks that the perivisceral fluid must be the chief medium for 

 the conveyance of oxygen to the tissues, and expresses the opinion 

 that the object of the peculiar arrangement of the cteca is to 

 bring about more effectually the oxygenation of their contents, 

 which are then, in Aphrodita, absorbed by the vessels of the true 

 vascular system. The contents of the cteca in that genus he 

 describes as consisting of a greenish chyle. Quatrefages has put 

 forward a similar theory as to the function of the creca, and 

 classes the Aphreditea, as regards their mode of respiration, with 

 the (Eohdce and Coslenierata as '* phlebenteriens." Jjo. Polynoe, 

 in which the relationship of the dorsal portion of the caeca 

 to the integument is by no means so close as in Aphrodita, in 

 which the integument of the dorsal tubercles and bases of the 

 parapodia is not in any marked degree thinner than that covering 

 the rest of the body, and in which there is no special arrangement 

 for driving a current of water over the dorsal surface, it is 

 probable that the process of respiration, though taking place in 

 part through the external skin, is also partly carried on by some 

 other means. It seems highly probable in fact that we have here 

 another instance of intestinal respiration, and it is quite possible 

 that the dorsal caeca may act, as the swimming-bladder is supposed 

 to act by Eisig, as a reservoir for oxygen or oxygenated water, 

 or may themselves act as respiratory organs. I have never found 

 these caeca to contain free gas, though bubbles may occasionally 

 be found in the intestine itself, so that it does not seem likely 



* Briliah Aunelidii, Koport of the British Asfjcciaticti; 18-31^ p. 200. 



