EGHINODERMA T A— RADIAL ORGANS 



409 



panying diagram will help to make this clear. The genus Ypsilo- 

 thurhi seems to have become fixed while in the act of being similarly 

 modified. 



In the genus Psychropotes (Fig. 223, p. 285) the dorsal surface is 

 prolonged beyond the anus into a long caudal appendage directed 

 posteriorly. Peniagone is distinguished by an anteriorly inclined comb, 

 rising transversely from the neck. On the swimming disc of Pelcujo- 

 thuria, cf. Figs. 224 and 225, p. 286. 



IV. Position and Arrangement of the Most Important Organs 

 in the Radii. 



The position and arrangement of the organs in the radii can best 

 be explained by describing cross -sections. In the Asteroidea, the 



Fig. 352.— Transverse Section of a radial region of the body wall of a Holothurian, 

 partly diagrammatic. 1, Endothelium of tlie body cavity ; 2, circular musculature ; 3, longitudinal 

 musculature ; 4, motor nerve ; 5, radial water vascular canal ; 6, radial blood lacuna ; 7, radial 

 ridge of the deeper oral nervous system ; 8, ampulla ; 9, cutis ; 10, epidermis ; 11, tube-foot canal 

 of the vascular system ; 12, tube-foot ; 13, nerve of the same ; 14, vessel of the same ; 15, radial 

 nerve strand of the superficial oral nervous system ; 10, epineural canal ; 17, i)priplieral nerve ; 18, 

 pseudohsemal canal. 



Ophiuroiclea, and the Crinoidea, in which the body is produced radially 

 into arms, the sections to be described will be those of the arms ; in 

 the Holothurioidea and Echinoidea the sections are of a radial region of 

 the body wall. 



Holothurioidea (Fig. 352). — In a transverse section through a 



