ECHINODERMATA 235 
stage is a mere thickened ridge of peritoneum, so that here, as 
in other Coelomata, the generative cells are derived from the 
lining of the coelom. 
The circumoral nerve ring, like the radial nerves, has lost 
its connection with the epidermis, and has sunk into the body. 
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Fic. 137.—A diagrammatic vertical section of an Ophiuroid, after Ludwig. The 
circumoral systems of organs are seen to the left, cut across, their radial pro- 
longations cut longitudinally, to the right. 
1. Body-wall. 101. External interradial muscle. 
2. Mouth. 10?. Internal interradial muscle. (The line 
3. Body-cavity. should point to the dotted tissue. ) 
31, Body-cavity of the arm. 11. Water-vascular system: to the left 
4, Oral ossicles. the circumoral ring, to the right the 
5. Torus angularis. radial vessel. 
6. Oral plate. 12. Polian vesicle. 
71. 1st ambulacral ossicle. 13. Nerve ring and radial nerve. 
7°, 7°, 74. 2nd to 4th ambulacral ossicle. 14. So-called blood-vessel. 
81, 8°, 8°. 1st to 3rd ventral plate. 14 (to the right). Genital rhachis enclosed 
9. 1st oral foot. in aboral sinus. 
10. Transverse muscle of the 2nd joint. 15. Radial perihaemal canal. 
The radial nerves in the arms are frequently segmented, a 
ganglionated swelling occurring corresponding with each ossicle. 
The generative organs consist of numerous caeca which 
open into a genital bursa. The bursae are ten in number, and 
le one on each side of each arm; they open ventrally by a slit- 
like aperture at the base of each arm. A genital rhachis con- 
nects the generative organs, which are surrounded by a 
blood-sinus, as in Asterids. Amphiura squamata is hermaph- 
rodite, and it is stated that when certain internal parasites, 
Orthonectidae, infest the coelom, it ceases to produce eggs, but 
produces a greater number of spermatozoa. 
Some of the Ophiuroids give off a phosphorescent light from 
the back of their arms. 
Ophiopholis bellis (Fig. 155) exists in great numbers in the 
