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 



CLOSURE IN OPHIURIDS AND 

 HOLOTHURIANS? 



CLOSURE IN ANTEDON 



0--ANUS OFCRINOIDS \. I, 



*"* -A-- ** 



/ \ ~~~~ANUS OF ASTERIDS 



/ POSTERIOR INTERRADIUS \ 



CLOSURE IN 

 ECHINIDS AND 

 HOLOTHURIANS? 



CLOSURE IN 



ASTERINA 



ANUS orExocvcLicA 



Bivium 



FIG. 83. Diagram of an Echiiioderm viewed from the aboral pole, to show the enumeration 

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

 hydrocoel closes in certain observed cases (vide Bury, Q.J.M.S., 29, p. 431). I-V the radii ; 

 M the madreporite. In MacBride's enumeration of the hydrocoel lobes adopted here in 

 the succeeding account of the development of Asterina, lobe no. 1 is radius II, lobe no. 2 

 is radius III, and lobe no. 5 is radius I. In Cuenot'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 III and V.I (Loven's plane of symmetry). 



