ECHINUS AMBULACRAL PLATES 



515 



surrounding the buccal tube-feet ; besides these, however, there 

 are in Echinus esculentus, and probably in most species, a large 

 number of thinner irregularly-scattered plates (Fig. 230). 



The term ambulacral plate, applied to the plate pierced by 

 the pores for the tube-feet, conveys a misleading comparison with 



comp.ret. 



comp.elv. 



tooth sac. 



Fig. 231. Dissection of Echinus esculentus. x 1. The animal has been opened by a cir- 

 cumferential cut separating a small piece of the skeleton at the aboral end, which is 

 turned outwards exposing the viscera on its inner surface. The other viscera are 

 seen through the hole thus made, amp, Ampullae of the tube-feet ; aur, auricle ; 

 b.v, so-called "dorsal blood-vessel" ; comp, "compasses" of Aristotle's lantern, 

 often termed " radii " by English authors ; comp.elv, elevator muscles of the com- 

 passes ; comp.ret, retractor muscles of the compasses ; eph, epiphyses of the jaws in 

 Aristotle's lantern ; gon, gonad ; g.rach, genital rachis ; int, intestine ; oe, oeso- 

 phagus ; prot> protractor of Aristotle's lantern ; red, rectum ; ret, retractor of 

 Aristotle's lantern ; siph, siphon ; st, stomach ; stone. c, stone-canal. 



the ambulacral plate of an Asteroid. In Echinoids the ambu- 

 lacral groove has become converted into a canal called the 

 "epineural canal," and the ambulacral plates form the floor, not 

 the roof, of this canal ; they may perhaps correspond with the 

 adambulacral plates of the Starfish, which one may imagine 

 to have become continually approximated as the groove became 

 narrower until they met. 



