Chap, xv.] EESPIEATORY OEGANS IN ANIMAL KINGDOM. 235 



the branchiae, which, in general, are \dsibly prolongations, 

 cutaneous a|>pendices. These appendices are sometimes simple 

 folds, sometimes foliated or pectiniform prolongations. They 

 are usually traversed by vessels whose blood afterwards returns 

 to the heart (Mgs. 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 



Schematic vertical sections of the types 

 pteropod (A) and cephalopod (B). The 

 heart is directed downwdxd. c, cepha- 

 lic part with indication in ^ of the 

 iins, which form part thereof, in £ of 

 the arms ; ^, intestinal canal ; &r, 

 liranchlECj p^ foot. 



Fio. 30. 



Folycera cristata, seen on 

 the dorsal side : — a, anal 

 orifice ; &r, branchise ; i, 

 tentacles. 



organs performing the work of respiration. The skin comes to 

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

 the respiration is wholly effected by the cutaneous surfac.e 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 



