ECHINODERMA GENERAL DESCRIPTION 



i.e. anterior, end of the archenteron becomes constricted off from 

 the rest of the archenteron to form the " coelom " (Fig. VI. 6). 

 The coelom sends backward a process on each side in the dorsal 

 region ; the hinder parts of these become constricted off as a 

 " right and left posterior coelom," which almost meet posteriorly ; 

 the remainder forms the " anterior coelom " (Fig. VI. 7). At the 



11 



Fie;. VI. 



Early stages of Echinoderm ontogeny. 1, 2, 3, the Echinoid Echinocyamus (after Theel). 

 4, 5, 6, the Crinoid Antedon (after Seeliger). 7, 8, 11, the Asteroid Asterina (after 

 MacBride). 9 (after Bury). 10 (after G. W. Field). 1, ovum in mucilaginous coat ; vitelline 

 membrane (v) beginning to separate ; s, spermatozoa, one of which is entering the yolk ( x 130). 

 2, segmenting OVUTII seen from above, 2 hrs. 20 min. after fertilisation. 3, blastula in 

 longitudinal section, 13 hrs. after fertilisation, c, cilia; sc, segmentation cavity (x200). 4, 

 gastrula, in section, 16 hrs.; ac, archenteron ; bp, blastopore (x73). 5, the same, 20 hrs., with 

 mesenchyme (ins) developing from endoderm. Letters as before (x88). 6, 48 hrs., with 

 blastopore closed, and archenteron constricted into ac anterior, mesenteron, and ec posterior 

 enterocoel (x88). 7, longitudinal section showing extension of right and left posterior 

 coeloms (rpc, Ipc) from anterior coelom around larval stomach (st), ( x 60). 8, further stage, 

 showing rpc and Ipc separated from ac, and lobes of the rudimentary left hydrocoel (he), (x60). 

 9, dorsal view of a Bipinnaria. oe, oesophagus ; hp, hydropore ; y, " blood-vascular space," 

 Bury, perhaps rudiment of right hydrocoel. Other letters as before ( x 50). 10, Bipinnaria, 

 of Astrrias, four days old. ce, ciliated band; m, mesenchymatous muscle fibres (x73). 11, 

 diagram showing relations of stone canal (stc) and pore canal (pc) to left hydrocoel (Ihc) and 

 anterior coelom (ac). 



hinder end of the anterior coelom, on both the right and left side, 

 there is a small outgrowth, the " right and left hydrocoel " (Fig. 

 VI. 8) ; that on the left is, as it happens, much more developed, 

 but the presence of a right hydrocoel has been proved by Metsch- 

 nikoff (1869) and MacBride (1896), (y in Fig. VI. 9). Near 

 the median dorsal line, above the hinder end of the anterior 

 coelom, a perforation arises in a thickening of the ectoderm, 



