EGHINUDERMATA — THE CQJLOMIC (J A VITIES 



441 



Hoors of these sacs consist of flat cells, but the epithelium round their openings into 

 the dorsal canal is much thickened, and provided with large cilia. These ciliated 

 sacs undoubtedly serve the same purpose as the ciliated bands in the brachial 



cavities of the Ophmroidm and the urns of the Synaptida: 



C. The Perioesophag-eal Sinus. 



1. Holothurioidea (Fig. 3G5, p. 428). — Between the mouth and 

 the water vascular ring the oesophagus is surrounded by a membrane- 

 like sheath, in such a Avay that between it and this membrane a 

 narrow cavity is left ; this is the pepioesophageal sinus, which is a 

 section of the coelom. The radial canals of the water vascular system 

 run along its outer side, and in the radial direction it is traversed by 

 numerous bands and filaments which are attached on the one hand to 

 the 03Sophageal wall, and on the other to the outer membrane of the 



Is 



Ri^ 



-10 9 



Fig. 369.— Median section through the oral region of Spatangus purpureus (after Cuenot). 

 Ig, Posterior uiiiiaired interradius ; Kini anterior unpaireil radius. 1, Kadial canal of tlie water 

 vascular system ; 2, radial blood vessel ; 3, radial pseudolufnial canal ; 4, radial nerve trunk ; 5, 

 radial epineural canal ; 6, test ; 7, body epithelium ; 8, mouth ; !i, epineural circular canal ; 10, 

 circular nerve ; 11, endothelium of the body cavity ; 12, perice soph age al sinus ; 13, water vascular 

 ring ; 14, blood vascular ring ; 15, oesophagus ; 16, membrane which separates the pericesophageal 

 sinus from the body cavity ; 17, septum, which separates the pseudohseinal canal from the periwso- 

 phageal sinus. 



sinus. These are only wanting in the part near the mouth, which is 

 thus distinct, and is known as the peribuccal sinus. The periceso- 

 phageal sinus is usually in open communication with the common bod}^ 

 cavity by means of a varying number of apertures in its outer mem- 

 brane. In Cucumaria there are five such apertures, which are large 

 and interradial in position. In the Elasipoda alone is the periceso- 

 phageal sinus completely separated from the general body cavity b}^ 

 an uninterrupted outer membrane, which runs from the water vascular 

 ring direct to the body Avail. 



2. Echinoidea. — In tlie Spatanguida (Fig. 369) a membrane, 

 which runs out horizontally from the commencement of the intestine 

 to surround the circular canal of the water vascular system, com{)letely 

 separates a very small pericesophageal sinus of the ccelom from the 

 sjjacious body cavity. In those Echinoids which are provided with a 

 masticatory framework, this latter develops within this sinus, Avhich 



