538 



INVEETEBEATA 



CHAP. 



IV—' 



the anterior pole of the larva, and so the transient asymmetry of the 

 metamorphosing period is removed, and the larva again attains 

 perfect symmetry. It is now in the form of a barrel surrounded by 

 five ciliated hoops, with the atrial opening at the anterior and the 

 anal opening at the posterior pole, and is known as the barrel- 

 shaped larva, or as the pupa. 



Whilst these external changes have been going on, the hydrocoele 

 ring has become completed by the union of the two ends of the hoop 

 which existed in the larva. During the process of the shrinkage of 

 the prae-oral part of the larva the hydrocoele hoop has rotated till its 

 plane, instead of being parallel to the length of the alimentary canal, 



becomes situated at right angles 

 to it. Then what was the posterior 

 and is now the ventral end of the 

 hoop grows under the oesophagus, 

 whilst the other end grows over 

 the oesophagus to the right, bends 

 down and meets the ventral end 

 beneath the oesophagus, and so 

 the ring is closed. Before actual 

 closure occurs the ventral end can 

 be seen to grow out into an in- 

 wardly directed lobe. This is the 

 rudiment of the Polian vesicle 

 {Pol, Fig. 401). 



The stone-canal opens into the 

 ring in the interspace between the 

 first and second of the smaller 

 lobes, that is, between rudiments of 

 the first and second radial canals, 

 counting as first lobe the one most 

 anteriorly situated, in the stage 

 where the plane of the hoop is 

 parallel to the long axis of the larva. 

 Therefore the stone-canal opens into 

 the ring in the same position as in the Asteroidea and Ophiuroidea. 

 With the closure of the ring the peri-oral coelom also forms a complete 

 circle, and the ventral horn of the left posterior coelom extends com- 

 pletely over to the right and round to the dorsal surface, where it 

 grows towards the main portion of the left posterior coelom, which 

 we may term its dorsal horn. The two horns are separated only by 

 the anterior coelom and its extensions, the stone-canal and the pore- 

 canal, just as is the case in Asteroidea, Ophiuroidea, and Echinoidea. 

 At the same time the oblique mesentery, separating left posterior 

 coelom and right posterior coelom, becomes largely broken down, and 

 this is also what occurs in Asteroidea. The longitudinal mesentery 

 of the adult is not, as Semon assumed, identical with the space 

 separating right and left posterior coelomio sacs in the larva, but 



Fig. 400. — Metamorphosing larva of 

 Synapta digitaia viewed from the 

 ventral aspect, (After Bury. ) 



Letters as in Figs. 398 and 39£l. In addition, 

 af, atrium : a portion of tlie larval stomodaeum ; 

 H, buccal tentacles ; cil.tr, transverse hoops 

 of cilia ; r.c, vestigial radial canals. 



