360 E. W. MACBRIDE. 
so-called heart, which projects along with the stone-canal into 
the axial sinus, was connected with this system, and that a 
string of tissue lying in the aboral ring and connected with the 
“heart”? was also part of the vascular system. We shall, 
however, see later that these two latter structures (“heart ” 
and aboral string) are of totally different nature from the oral 
ring, being composed of primitive germ-cells, and have, as a 
matter of fact, no connection with it. ‘The radial tracts are 
absent in Asterina, but the oral circular tract is well repre- 
sented, and we shall study its development later. 
The woodcut shows us that the foregoing description is not 
quite correct. In the first place, we see that one can hardly 
speak of an outer perihemal ring, because this space is broken 
up into five compartments by the prolongations of the longi- 
tudinal septa of the radial canals; secondly, apart from the 
mistake we just pointed out in reference to the nature of the 
‘heart’ and aboral ring, we see that the axial sinus (a’) does 
not open into the perihemal aboral ring ; and, further, that to 
the upper end of the axial sinus is closely apposed a small 
closed sac, the right hydroceele. 
Returning to figs. 51—53, we see that each of the five 
compartments of the outer oral perihemal ring arises 
separately as a wedge-shaped outgrowth of the 
ccelom. Ihave numbered these rudiments according to the 
numbers of the lobes of the hydroceele between which they 
occur—ph. 1.2, ph. 2.8, ph. 3.4, ph. 4.5, and ph. 5.1; the last, 
however, arises later, and is not seen in these figures, and the 
first is an outgrowth of the anterior coelom (Pl. 20, fig. 51, Pl. 
21, fig. 54) : all the rest arise from the left posterior celom. The 
shape and relations of these rudiments are well shown in the 
enlarged drawing given of one of them (Pl. 27, fig. 139) ; we 
see that the base of the wedge is directed outwards, and that its 
basal angles tend to insinuate themselves between the ectoderm 
and the hydrocele. As a matter of fact, each angle grows out 
till it meets the adjacent one of the next rudiment. The two 
then become apposed to each other, and their walls, which 
meet, form the longitudinal septum of the radial canal, and 
