120 



PHYLUM ECHINODERMATA. 



spicules of Holothurians, which are remarkable for their definite 

 and symmetrical form, to the compact and complete dermal 

 skeleton of Echinoids. The plates arise as deposits in the form 



Trivium 



r 



in 



CLOSURE IN OPHIURIDS AND 

 HOLOTHURIANS ? 



CLOSURE IN ANTEDON 



IV 



CLOSURE IN 

 ECHINIDS AND 

 HOLOTHURIANS ? 



POSTERIOR INTERRADIUS 



CLOSURE IN 

 ASTERINA 



ANUS OF EXOCYCLICA 



Bivium 



FlO. 83. Diagram uf an Kchinoderm viewed from the aboral pole, to show the enumeration 

 of the radii adopted in this work. The diagram also shous the interradius in which the 

 liydrocoel closes in certain observed eases (vide Bury. (J..I.M.S.. '2'.1. p. 431). I-V the radii ; 

 M the inadrejiorite. In MacBride's enumeration of the liydrocoel lolies adopted hen 1 in 

 the succeeding account of the development of Asterhui, lulie no. 1 is radius II. lolie no. ~ 

 is radius III, ami lolic no. f> is radius I. In Cue-not 's enumeration the equivalence is A = V. 

 B = I, C = II, D = III. and E = IV. Radius I is the right anterior, and radius IV is the left 

 anterior radius of Crinoids. The plane of symmetry of Holothurians and Crinoids passes 

 through radius V and interradius II. Ill, that of Asteroids through IV and I. II. and that 

 of exocyclic Echinoids through 111 and V.I (Loven's plane of symmetry). 



