DEVELOPMENT OF ANTEDON 619 



anterior division divides into a thick-walled sac, the hydrocoel, 

 on the left, and a median thin-walled anterior coelom, which 

 sends a long extension into the anterior portion of the larva, 

 which we may compare to the prae-oral lobe of the Bipinnaria. 

 The anterior coelom communicates with the exterior by a short 

 pore-canal, and later forms a connexion, the stone-canal, with the 

 hydrocoel. At . the apex of the prae-oral lobe there is formed 

 a thickened patch of ectoderm, bearing stiff sensory hairs, and 

 having at their bases nerve -fibres and ganglion cells. This 

 larval brain corresponds to that of the Tornaria and Echino- 

 pluteus. Behind the brain there is a glandular pit, which is 

 used for fixation, and recalls the similar organ in the Bipinnaria. 

 A series of ciliated rings is then formed, and between the second 

 and third of them an oval depression appears. This is the 

 stomodaeum; but as the larva takes no food it does not com- 

 municate with the gut (Fig. 295, A, stoDi). 



The larva next escapes from the egg-membrane and swims 

 freely for a day or two, and then, like the Bipinnaria, fixes 

 itself by the apex of the prae-oral lobe, which is converted into a 

 stalk. The larval stomodaeum closes, and the oesophagus of the 

 adult appears as a solid peg of cells abutting against it ; round 

 this peg the hydrocoel grows like a ring. 



The closed stomodaeum and the underlying hydrocoel are 

 now rotated backwards until they come to be at the end of 

 the animal opposite the stalk (Fig. 295, C). The left posterior 

 coelom, which has also, as in the Asteroid larva, assumed a hoop- 

 like form, is carried along with them ; but the right posterior 

 coelom becomes shifted forwards and sends out five outgrowths 

 into the stalk, which form the rudiments of the chambered organ, 

 and a central one as a continuation of the genital stolon (Fig. 

 295, D, gen.st), the extension of the anterior coelom (Fig. 294, B) 

 having disappeared. 



Then the outer wall of the stomodaeum splits into five 

 valves — the future oral valves. The radial canals appear as 

 freely projecting tentacles, which issue in the intervals of these 

 valves and soon acquire two pairs of lateral branches. The 

 skeleton consists of five oral plates in the oral valves, of a ring 

 of five basals, of three small under -basals, and of a series of 

 "columnals," i.e. stem -ossicles, as rings embracing the stalk. 

 The area of attachment is supported by a " foot-plate." The radial 



