XVI 



ECHINODEEMATA 



497 



to this extent the development of an Ophiuroid is more primitive 

 than that of an Asteroid. 



Stage C. 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 



st p.o.c Ipc 



sic I germ 

 'mp 

 amp 



ten 



m.v 



ep 



nr 



. 380. 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 Ophiothrix fragilis juat after metamorphosis. B, section through the 

 embryo of Auipltium squumata '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 ; 

 J.p.i; left posterior coelom; m.p, madreporic pore; m.r, madreporic vesicle; n.r, nerve ring ; ph. I. '2, 

 pcrihaemal space, derived from the left anterior coelom (ampulla) and still opening into it ; p. <>.<, peri- 

 oral coelom ; p. <<, 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 



VOL. I 2 K 



