CHAP, xv.] RESPIRATORY ORGANS IN ANIMAL KINGDOM. 235 



the branchiae, which, in general, are visibly prolongations, 

 cutaneous appendices. These appendices are sometimes simple 

 folds, sometimes foliated or pectiniform prolongations. They 

 are usually traversed by vessels whose blood afterwards returns 

 to the heart (Figs. 29 and 30). When the vessels are lacking 

 there are at least lacunar cavities. But though these branchise 

 are visibly respiratory organs, they are far from being the only 



FIG. 29. 



Schematic vertical sections of the types 

 pteropod (A) and cepkdlopod (B). The 

 heart is directed downward, c, cepha- 

 lic part with indication in A of the 

 fins, which form part thereof, in B of 

 the arms ; tr, intestinal canal ; br, 

 branchiae, p, foot. 



FIG. 30. 



Polyce.ro, cristate, seen on 

 the dorsal side : a. anal 

 orifice; br, branchiae; t, 

 tentacles. 



j organs performing the work of respiration. The skin comes to 



! their aid and respires also. Sometimes there are no branchise ; 



1 the respiration is wholly effected by the cutaneous surface and 



by the intestinal canal. This last surface is, moreover, clothed 



with a vibratile epithelium, like that of the branchise, and the 



oscillatory movements of these cilia are continued from the anus 



to the stomach. The anus opens and shuts by a rhythmical 



