VIII 



EGHINODERMATA— RADIAL ORGANS 



409 



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

 thuiia 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. Feniagone is distinguished by an anteriorly inclined comb, 

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

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



IV. Position and APFangement of the Most Important Organs 

 in the Radii. 



The position and arrangement of the organs in the I'adii can best 

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



Fio. 3.52.— Transversa Section of a radial region of tlie body wall ot a Holotliurian, 

 partly diagrammatic. 1, Bndotlielium ot the body cavity ; 2, circular musculature ; S, lougitudiual 

 uiusculature ; 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 systein ; 13, tube-foot ; 13, nerve of the same ; 14, vessel of the same ; 16, radial 

 nerve strand of the superficial oral nervous system; 16, epineural canal ; 17, peripheral nerve ; 18, 

 pseudohsemal canal. 



Ophiuroidea, 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 



