XVI 



ECHINODERMATA 



549 



will be seen that the longitudinal mesentery separating the left and 

 right posterior coeloms runs in a curve, the concavity of which is 

 directed forwards. 



During free-swimming life the hydrocoele, which lies in a concavity 

 ot the gut, takes on a hoop-like form with the concavity directed 

 towards the left ; later it rotates so that the concavity is directed 

 torwards, and it is then seen that the right limb is thicker than the 

 left. The five primary lobes, the rudiments of the future radial 



Fia. 405. — External views of larvae of Antedon rosoxea — at the time of hatching and 

 after fixation, in order to show the development of the calcareous ossicles. (After Seeliger. ) 



A, larva at the time of hatching. B, larva two days after hatching— fixed. Letters as in previous 

 figure. In addition, col, rudiments of the columnal plates ; Bi-B^, rudiments of the five basal plates ; 

 /.j)j foot-plate ; 01-05, rudiments of the five oral plates ; SB, rudiments of the under-basal plates. 



canals, arise as ventrally directed outgrowths. The stomodaeum 

 closes behind so that its opening becomes narrowed. The right 

 posterior coelom sends forward two dorsal diverticula which lie above 

 the anterior coelom. These two fuse with one another ventrally, but 

 their lateral walls, where they are in contact with one another, form 

 the longitudinal mesentery of the right coelom. From their apices 

 spring five narrow diverticula whose openings are arranged in a circle; 

 these extend forwards to the apical field and constitute the rudiment 

 of the chambered organ (Fig. 406, B). The left coelom extends so far 



