902 



ECHINODERMA 



beginning to enlarge, to form the centro-dorsal plate (2, c d), from 

 which are beginning to spring the dorsal cirrhi (cir) that serve to 



FIG. 686. Pentacrinoid larva of Antedon. 1. Skeleton of early pentacrinoid, 

 under black-ground illumination, showing its component plates : b, b, 

 basals, articulated below to the highest point of the stem; r l , r 1 , first 

 radials, between two of which is seen the single anal plate, a ; r ! , second 

 radials ; r 5 , third radials, giving off the bifurcating arms at their summit ; 

 o, o, orals. 2, 3. Back and front views of a more advanced pentacrinoid, 

 as seen by incident light, one of the pair of arms being cut away in fig. 8 

 in order to bring the mouth and its surrounding parts into view : b, b, 

 basals ; r 1 , r 2 , r 5 , first, second, and third radials ; a, anal, now carried 

 upwards by the projection of the vent, v ; o, o, orals; cir, dorsal cirrhi, 

 developed from the highest joint of the stem. 



