242 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 (p. 241) shows us that the foregoing description 

 is not quite correct. In the first place, we see that one can hardly 

 speak of an outer periheemal 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 perihsemal aboral ring ; and, further, that to 

 the upper end of the axial sinus is closely apposed a small 

 closed sac, the right hydrocoele. 



Returning to figs. 51 — 53, we see that each of the five 

 compartments of the outer oral perihsemal ring arises 

 separately as a wedge-shaped outgrowth of the 

 coelom. 1 have numbered these rudiments according to the 

 numbers of the lobes of the hydrocoele between which they 

 occur — ph. 1.2, ph. 2.3, 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 (PI. XIII, fig. 51, 

 PI. XIV, fig. 54): all the rest arise from the left posterior coelom. 

 The shape and relations of these rudiments are well shown in 

 the enlarged drawing given of one of them (PI. XX, fig. 139) ; 

 we see that the base of the wedge is directed outwards, and that 

 its basal angles tend to insinuate themselves between the ecto- 

 derm and the hydrocoele. 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 



