XVI 



ECHINODEEMATA 



497 



to this extent the development of an Ophiuroid is more primitive 

 than that of an Asteroid. 



Stage G. — In the next and concluding stage of the meta- 

 morphosis, which we may designate stage C, all trace of the larval 

 arms, except the two postero-lateral, has disappeared. The adult arms 

 have, however, grown in length and have become apposed to the lobes 

 of the hydrocoele. This apposition is brought about by the shrinkage 



Fig. 380.- 



ten 



-Transverse sections through the discs of two young brittle-stars in order to 

 show the origin of the germ cells. (Original.) 



A, section through young Ophiotlirix fragilis just after metamorphosis. B, section through the 

 embryo of AmpMura squamata -25 mm. across the disc taken from the maternal brood pouch, amp, 

 ampulla, the derivative of the left anterior coelom ; ep, epineural space ; germ, primitive germ cells ; 

 l.p.c, left posterior coelom ; m.p, madreporic pore ; m.v, madreporic vesicle ; n.r, nerve ring ; ph.1.2, 

 perihaemal space, derived from the left anterior coelom (ampulla) and still opening into it ; p.o.c, peri- 

 oral coelom ; p.vcw, posterior vacuolated crest ; st, stomach ; st.c, stone-canal. 



of the ectoderm connecting the two structures. The outline of the 

 young Ophiuroid, instead of being merely pentagonal, as it was in 

 the preceding stage, has become definitely five-rayed, but the rays 

 are folded inwards underneath the disc. 



There is found as an outgrowth of the left posterior coelom, a 

 peri-oral coelom (p.o.c, Fig. 380), corresponding to that found in 

 Asteroids, which interposes itself between the left posterior coelom 

 and the oesophagus. This appears to persist throughout life in the 



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