76 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



1. Notice that the layer of epithelium which forms the 

 outer surface of the coeca passes down the mesenteries at 

 r, and is continuous with that which lines the body wall. 



2. The coeca are lined by another layer (e). 



3. Notice that each mesentery is double, and encloses 

 a space which is lined by another layer of epithelium. 

 If a dissected specimen be examined, this space (s) will 

 be found to open into the peri visceral cavity at the base of 

 the arm, where the coeca give rise to their ducts. The 

 lining epithelium is therefore a continuation of the 

 layer (c). 



i. Notice the cross section of the radiating water- 

 tube (7), in the angle between the two ambulacra! 

 plates (q). 



1. Notice the small tubes which connect this tube with 

 the ambulacra. 



2. Notice the tube which passes through the ambolacral 

 pore, between the anil mineral plates, and connects the 

 ambulacral vesicle (A 1 ) with the foot (). 



3. The ambulacral vesicle is made up of three 

 layers : 



(i.) A lining epithelium (d) continuous with the lining 

 of the water-tube and foot. 



(ii.) An outer layer continuous with that which lines 

 the perivisceral cavity. 



(iii.) A layer of muscular fibres between the two. 



4. Notice that the foot (n) consists of three similar 

 layers, but the outer one is continuous with the layer (a) 

 which invests the outer surface of the ray. 



j. Notice that the layer (a) is greatly thickened on the 

 middle line of the ray under the water-tube, to form the 

 radiating nerve (m). 



k. Between this and the water-tube there is a space (?<) 



