244 ZOOLOGY 
The radial nerves send branches to the tube-feet, from the base 
of which a nerve passes to the sub-epidermal plexus of nerve 
LLL 
Ze 
Fic. 146.—Diagrammatic vertical section of a Sea Urchin (from Leuckart), 
After Hamann. 
1. Mouth. 10. Aboral sinus containing so-called 
2. Intestine cut short. blood-vessel. 
5. Siphon. 11. Circumoral water-vascular ring. 
4. Rectum. 12. Oral nerve ring. 
5. Anus. 3. Tube-foot with ampulla. 
6. Ventral vessel on intestine. 14. Radial nerve. 
7. Dorsal vessel on intestine. 15. Radial water-vascular vessel. 
8. Stone canal. 16. Polian vesicle. 
9. Madreporic plate. 17. Muscles. 
18. Ocular plate. 
fibrils, which ramify all over the body just outside the calci- 
fications, and govern the movement of the pedicellariae and 
spines. : 
The generative organs typically consist of five arborescent 
glands, though the number is often reduced, lying interradially, 
and opening on the genital plates. In the young they are all 
connected by a circular genital rhachis; they become very 
conspicuous in the breeding season. 
The pore plates of the paired ambulacral areas are in the 
female Hemiaster philippii extended and depressed so as to 
form four deep oval cups. In these the eggs are deposited 
