234 



ZOOLOGY 



is produced into five radial and five short interradial caeca. 

 The walls of the stomach are lined by a ciliated epithelium. 



Fio. 136. — Diagram of a transverse 

 sectioiL of an Ophiuroid, 



1. Eadial nerve, witli lateral 



branches. 



2. So-called radial blood-vessel. 



3. Eadial -water-vascular trunk. 



4. Tube-foot. 



5. Ventral plate. 



6. Lateral plate. 



7. Ambulacra! ossicles. 



8. Dorsal plate. 



9. Dorsal portion of coelom. 



10. Muscles. 



11. Lateral nerve. 



12. Origin of lateral nerve. 



and are supported by connective tissue strands, which traverse 

 the coelom to the body-wall. There is no anus. 



The water-vascular system consists of a circumoral ring, 

 which bears four Polian vesicles ; in the fifth interradius it 

 gives off the ciliated stone canal, which is simple and un- 

 calcified, this passes to the madreporic plate on the ventral 

 surface. In Astrophyton there are five madreporic plates, one 

 in each interradius, and five stone canals. The radial vessels 

 which arise from the ring bear no ampullae, but give off 

 branches which pass directly to the conical tube-feet. Cor- 

 puscles tinged with haemoglobin occur in the water-vascular 

 fluid of one species. 



The true vascular system resembles that of Asterids. The 

 aboral ring has, however, an undulatory curve, being ventral 

 in the interradii. MacBride has recently proved that both the 

 axial sinus and the aboral ring are involutions of the coelom. 

 The so-called heart is nothing but a genital stolon, whence the 

 genital rhachis grows out. The genital stolon in the earliest 



