480 



INVEKTEBEATA 



CHAP. 



plates which make up the skeleton of a Crinoid, the value of which 

 will be examined when we consider the development of a Crinoid. 



The origin of the genital organs is very peculiar. About the 

 time that the metamorphosis is completed a peculiar fold appears 

 in the wall of the axial sinus which abuts against the left posterior, 



mu 



uxur 



abs 



© 



gear g^^si: 

 mu 



Fig. 371. — Three figures to illustrate the development of the genital stolon, genital rachis, 

 and gonad in Asterina gibbosa. (Original.) 



A, vertical section of the disc of a specimen that has just metamorphosed. Disc about '75 mm. in 

 diameter. B, horizontal section through one of the inter-radial septa of a specimen about 1-2 mm. across 

 the disc. C, Section through the incipient gonad of a specimen about 6 mva. across the disc, ab.s, 

 aboral sinus ; aici, axial sinus ; rf.o, dorsal organ ; (jen.r, genital rachis ; gen.st, genital stolon ; gon, 

 gonad ; gon.s, gonadial sinus ; m.v, madreporic vesicle ; ph, perihaemal space ; pr.g.inVi primitive 

 germinal involution ; st.c, stone-canal. 



or, as we may now term it, the hypogastric coelom. This fold is the 

 rudiment of the strand called axial organ, dorsal organ, or genital 



stolon, and formerly regarded as a heart {d.o, Fig. 371, A). At the 

 dorsal edge of the hypogastric coelom, where the remains of the 

 mesentery which separated it from the epigastric coelom are still to 

 be seen, an involution of the coelomic epithelium takes place which 



