82 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



p. Cut a section through the mouth-pentagon, along the 

 axis of a ray, and comparing it with the transverse section 

 of the ray, and with the one through the mouth-pentagon 

 in the plane of an inter-radius, notice : 



1. The ambulacral plates (Fig. 35, a m, a TO'). 



2. The peristome (p). 



3. The layer of epithelium (h) which lines the perivis- 

 ceral cavity (A). 



4. The radiating w T ater-tube (r w) running out from the 

 circum-oral water-tube (c w). 



5. The muscles (m, m', m") which run across the ambu- 

 lacral furrow, from one ambulacral plate to the other, un- 

 derneath the water-tube. Notice that while there are two 

 of these muscles (m and m') for the. first ambulacral 

 plate (am) there is only one for each of the others. The 

 fact that the duct which joins the first ambulacral vesicle 

 to its foot passes through the first ambulacral plate instead 

 of between it and the second has been already noticed. 

 This and the presence of two muscles instead of one indi- 

 cate that this plate really represents two of the series 

 fused together instead of a single plate. 



6. The central channel (a) Of the circum-oral peri- 

 haemal vessel, in the same position as in the section along 

 an inter-radius. 



7. The partition which separates this from the outer 

 channel (c). 



8. The blood-vessels in this partition. 



9. The radiating blood-vessel (d) which runs from this 

 partition to the tip of the ray. 



10. The partition (A 1 ) which runs along the middle of 

 the ray and supports this vessel between the halves of the 

 radiating peri-hremal vessel. Notice that the halves of 

 the radiating peri-haemal vessel communicate with each 



