500 



INVEETEBRATA 



CHAP. 



While these developments are going on in the skeleton the 

 foundations of the genital system are being laid. The development 

 of this has also been worked out by us (MacBride, 1892). It will be 

 remembered that the madreporic pore, and of course the pore-canal and 

 left anterior coelom along with it, are forced from a marginal to a 

 ventral position owing to the growth of the iuterradius. The original 

 mesentery separating oral from aboral coeloms, i.e. left from right 

 posterior coelom, is largely perforated, but it too persists and extends 

 from the stomach obliquely downwards instead of upwards. The stone- 

 canal also extends obliquely downwards, and where the wall of the 



a l c l 



FIG. 381. Two diagrams to elucidate the mutual relationships of stone-canal, dorsal organ, 

 axial sinus, madreporic vesicle, genital rachis, etc., in an Asteroid and an Ophiuroid 

 respectively. 



A, relations of these organs in an Asteroid. B, relations of these organs in an Ophiuroid. db.s, 

 aboral sinus ; a'ci, space derived from the anterior coelom in A termed axial sinus, in B ampulla ; d.o, 

 dorsal organ ; gen.r, genital rachis ; m.p, madreporic pore ; m.v, madreporic vesicle ; p.g.inv, primitive 

 germinal invagination this space disappears in A, but in B forms the so-called axial sinus ; st.c, stone- 

 canal. The arrow shows the direction in which the structures in A must be rotated to reach the con- 

 dition in B. 



left posterior coelom abuts on it one can observe that the nuclei 

 of the latter become enlarged. 



This is the first indication of the primitive germ-cells, and forms 

 the genital stolon tissue, constituting the dorsal organ. It becomes 

 protected by the outgrowth of a covering flap which grows down from 

 the central end of the stone -canal. The space left between the 

 primitive germ-cells and the cover-flap has been confused with the 

 axial sinus of Asteroidea, and has received the same name ; it is, 

 however, quite distinct from this, and corresponds to the cavity 

 of the primitive germinal involution of Asteroidea, which soon 

 closes ; the corresponding cavity in Ophiurids persists for life. 



