ECHINODERMA TA 



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. 



V 8* 10 6» 



Fig. 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. 



2. Month. 



3. Body-cavity. 



3\ Body-cavity of the arm. 



4. Oral ossicles. 



5. Torus angularis. 



6. Oral plate. 



7^. 1st ambulacral ossicle. 



1^, V, 7*. 2nd to 4th ambulacral ossicle. 



8\ 8^ 8^ 1st to 3rd ventral plate. 



9. 1st oral foot. 

 10. Transverse muscle of the 2nd joint. 



10^. External interradial muscle. 

 10^. Internal inten'adial muscle. (The line 

 should point to the dotted tissue. ) 

 Water - vascular system: to the left 

 the circumoral ring, to the right the 

 radial vessel. 

 Polian vesicle. 



Nerve ring and radial nerve. 

 So-called blood-vessel. 

 14 {to the right). Genital rhachis enclosed 



in aboral sinus. 

 15. Radial perihaemal canal. 



11 



12. 

 13. 



14. 



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 

 lie 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. Aviphiwa 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. 



OpMopholis bellis (Fig. 135) exists in great numbers in the 



