488 



INVEETEBEATA 



CHAP. 



passive stretching of the ectoderm, but rather because the pressure of 

 the growing tip of the rod stimulates the ectoderm to increased growth. 

 If the larva be exposed to unfavourable conditions, such as lack of 

 oxygen, etc., the ectoderm shrinks, and the pointed tip of the 

 calcareous rod projects as a naked spine, so that if there were any- 

 stretching of the ectoderm the spine would immediately pierce it. 

 As the anterior vacuolated crest disappears, another, consisting of 



Fig. 375. — Ventral views of young larvae of Ophiothrix fragilis. (Original.) 



A, larva, two and a quarter days old. B, larva, three days old. a, anus ; ct.Z.a, antero-lateral arm ; 

 a.i.r, antero-lateral rod ; &.r, body rod ; cU.h-ng, longitudinal ciliated band ; int, intestine ; l.coe, left 

 coelomic sac; p.l.a, postero-lateral arm; p.l.r, postero-lateral rod; p.o.a, post-oral arm; p.o.r, post- 

 oral rod ; r.coe, right coelomic sac ; H, stomach ; stom, stomodaeum. 



a precisely similar ridge of ectoderm, makes its appearance at the 

 posterior pole of the larva, and this second crest persists through 

 the entire period of larval development. This we may term 

 the posterior vacuolated crest. The purpose of these crests 

 seems to be to diminish the specific gravity of the larva, and so 

 balance the increase in weight due to the development of calcareous 

 matter. The posterior rod of the calcareous star on each side meets 

 its fellow just above this posterior crest. The lower ends of 

 these "body-rods" bifurcate, and, by the meeting of the forks 



