260 E. W. MACBRIDE. 



Comparing figs. Ill aud 112 we see that the axial sinus of 

 Asterina is represented in Amphiura by sinus c, the so-called 

 "ampulla/' The aboral sinus {ab, fig. Ill, sinus a, fig. 113) 

 is also obviously homologous in both. 



[Since my paper (14) was published, and since the present 

 work was sent in for publication, I have made a careful re- 

 examination of my sections of Amphiura squamata, and 

 have arrived at a more complete comprehension of the structure 

 and development of the ovoid gland and the neighbouring 

 spaces in that animal. The space marked sinus b' (fig. 112) 

 is not, as I formerly supposed, a part of sinus b, but is quite 

 distinct. Sinus b' probably represents the right hydrocoele; 

 it is already present in the youngest specimens I examined. 

 Sinus Z>* is a portion of the coelom shut oS by the outgrowth of 

 a flap of peritoneum ; from the inner wall of this sinus the 

 cells which at the same time give rise to the ovoid gland and 

 to the genital rachis take their origin ; it is obviously homolo- 

 gous to the cavity of the invagination of the primitive germ 

 cells {pr. germ inv., figs. 110 and 111), only in Asterina this 

 space disappears. — December, 1895.] 



We observe that the arrangement in Amphiura might be 

 obtained from that in Asterina by rotating the stone-canal and 

 accompanying structures outwards and downwards through an 

 angle of 180°. That this is what has occurred in phylogeny 

 is indicated, not only by the fact that in the young Amphiura 

 the madreporite is near the edge of the disc and the stone- 

 canal almost horizontal, whereas in the adult the madreporite 

 is situated far in towards the mouth on the oral surface, but 

 also by the curious undulating course of the genital rachis, 

 which is aboral in the interradii and oral in the radii. This 

 points to the conclusion that the aboral parts of the interradii 



* In my paper on tliis subject (14) sinus 1/ is referred to as the axial sinus — 

 it was formerly supposed to be continuous with sinus c, though Ludwig knew 

 this was not so. At tiiat time the meaning of the axial sinus in Asterids 

 which Bury first suggested (2) was not generally known, and his interpreta- 

 tions were not accepted, and hence two different spaces were called axial 

 sinus, one iu Asterids and the other in Ophiurids. 



