CEINOIDEA. 285 



The alimentary canal is tubular and passes from the central 

 or subcentral mouth to the excentric anus. In this passage it 

 executes, in Antedon, one complete coil in the calyx. Its lining 

 is ciliated and its central portion is slightly dilated. In some 

 forms caecal outgrowths of the canal are present. 



In Actinometra the digestive tube executes four complete coils in the 

 calyx before the anus is reached. Moreover the mouth is excentric, being 

 shifted anteriorly, while the anus is subcentral. 



The coelom presents the usual division into perivisceral cavity 

 and water- vascular system. 



The perivisceral cavity occupies the calyx and extends into 

 the arms. The calycine portion is for the most part traversed 

 by a connective tissue network in which calcareous structures 

 may be present. In some Comatulidae three parts of the body 

 cavity may be distinguished. (1) an axial portion, in which 

 there are no connective tissue strands ; this occupies the axis 

 of the calyx ; (2) a perivisceral portion around the gut coils, and 

 (3) a subcutaneous portion just beneath the integument and 

 marked off from (2) by a kind of septum. These parts do not 

 appear to have any special importance. 



The perivisceral cavity is continued into the arms as three 

 distinct sets of cavities, which however communicate at inter- 

 vals. These are (Fig. 197), (1) the dorsal or coeliac canal (7), 

 {2) the canal containing the generative rachis (6), and (3) the 

 ventral or sub tentacular canal (-5). The last is divided by a 

 septum into two, and on the dorsal wall of the dorsal canal de- 

 pressed patches of ciliated epithelium are occasionally met with, 

 especially in the pinnules. All these parts of the body-cavity 

 are continued into the pinnules (Fig. 198), the only difference 

 being that in the pinnules the generative rachis is swollen up 

 into the generative glands. 



The ventral canal, on reaching the calyx, opens into the 

 axial part of the perivisceral space. The genital section is lost 

 in the meshes of the body-cavity round the oesophagus, and 

 the dorsal canal opens into the subcutaneous part of the body- 

 cavity of the calyx. There is nothing corresponding to the 

 axial sinus of Asterids and Echinids. 



There is in the arms and pinnules a canal * (2) between the 



* This canal is not always distinguishable. It may be due to shrinkage. 



