6o8 



ECHINODERMATA 



however, as in the Bipinuaria, separated from the rest of the 

 band. The processes of the band are much more faintly marked 

 than in the Bipinnaria, the anterior median, prae-oral, and 

 median dorsal processes being absent ; but a pair of intermediate 

 dorsal processes are developed in the- interspace between anterior 

 and posterior dorsal. 



In the Bipinnaria, Ophiopluteus, and Echinopluteus the 

 coelomic vesicle, after separation from the archenteron, divides 

 into right and left halves. The left then sends out a short. 



Fig. 286. — Three views of metamorphosis of Auricularia of Synapta digitata. A, fully 

 grown xUiricularia ; B and C, stages in the metamorphosis, hy, Sydrocoel ; Int, 

 intestine ; l.px, left posterior ooelom ; 0, fragments of ciliated tand which are 

 invaginated into the stomodaeum, and coalesce to form a ring round the mouth ;, 

 OSS, ossicle ; pod, rudiment of feelers which here spring directly from the hydrocoel ; 

 T.p.e, right posterior coolom ; st, stomach ; w.v.r, rudiment of water-vascular radial 

 canals ; 1-6, corresponding pieces in the three figures of the longitudinal ciliated 

 band. (After Bury.) x 40. 



dorsal process, which, fusing with the ectoderm, acquires an 

 opening to the exterior. This opening is the primary madreporic 

 pore, and the process of the left coelomic sac, which is ciliated, 

 is the pore-canal. In the Auricularia the pore and ]pore-canal 

 are formed lefore the division of the coelom. In the Bipinnaria 

 the right and left sacs subsequently fuse in the front part of the 

 prae-oral lobe. In the first three types of larva the coelomic sac on 

 each side then undergoes, a segmentation into anterior and posterior 

 portions. At the hinder end of the anterior sac on each side a 

 swelling occurs. That on the left side is the " hydrocoel," or 

 rudiment of the water- vascular system (Fig. 287, A^?.%); it 



