260 PHYLUM ECHINODERMATA. 



formed from the respiratory trees. The branched cuvierian organs have 

 not got the viscid wall and their function is not understood. 



The coelom presents the usual main divisions, viz. (1) the am- 

 bulacral system (2) the perivisceral cavity, and (3) the perihae- 

 mal spaces. The ambulacra! system has already been described. 



The perivisceral cavity is spacious and is traversed by the dorsal 

 (larval) mesentery (see p. 257), and by muscular and connective 

 strands. It is lined by a ciliated epithelium and contains a 

 corpusculated fluid. It has so far as is known no communica- 

 tion with the exterior, but in many Holothurians it communicates 

 with the water-vascular system through the so-called inner 

 madreporite. 



There is a special section of the body-cavity round the oesophagus, 

 called the perioesophageal sinus (Fig. 179, 8, 11). This is separated from 

 the rest of the body-cavity by a membrane, which however is in most 

 cases perforated by apertures putting it in communication with the general 

 body-cavity, and it is tran versed by strands of tissue passing from the 

 wall of the oesophagus to this outer wall of the sinus, except just round 

 the mouth opening where it is free from trabeculae and constitutes the 

 peribuccal sinus. In the Elasipodidae the outer membrane is complete 

 and the perioesophageal sinus is completely shut off from the rest of the 

 body-cavity. 



In the Synapticlae there is a number of funnel-shaped ciliated organs 

 attached by a stalk to the mesentery and body wall. The cavity of these 

 organs, which is freely open to the body cavity, is lined by a ciliated 

 epithelium and ends blindly in the stalk. The stalk may be branched and 

 carry many ciliated funnels. 



The perihaemal spaces, which are almost certainly parts of 

 the coelom, are lined by a flat epithelium and contain a fluid 

 similar to that of the body-cavity. They consist of five radial 

 canals (Fig. 180, Sin.rd) placed just internal to the radial nerves, 

 between these and the radial blood-vessels. A circumoral peri- 

 haemal canal has been described in some cases (Synapta, 

 Cucumaria, etc.). 



In the Synaptidae the perihaemal canals do not extend very far from 

 the oral region, and the circumoral perihaemal space is separated by a 

 septum from the radial perihaemal spaces. The perihaemal spaces appear 

 not to communicate with the perivisceral cavity. 



The vascular system. Immediately aborad (Fig. 179, 14) 

 of the water-vascular ring is the circumoral vessel of the so- 

 called vascular system. This sends off to each radius a vessel 

 which extends to the aboral end of the body just external to the 

 water- vascular canal (Fig. 180, lac.rd). It is also connected 



