174 P, HERBERT CARPENTER, 
Sharpey to be merely the spaces between the two folds of 
mesentery in which each pyloric cecum is slung (fig. 3, m.). 
They are only partially separated portions of the general 
body cavity with which they are connected in the disc. 
Greeff, Hoffmann, and Teuscher were able to inject the 
tubular space enclosing the “heart”? and stone-canal from 
this aboral ring described by Tiedemann; and Greeff 
described the lumen of the ring as partially filled up by a 
hollow fold, which exhibits the same structural characters 
as the “heart” and the gastric vascular bundles. As in 
the case of the oral ring, the hollow fold discovered by 
Greeff in Tiedemann’s anal ring has been shown by Ludwig 
to constitute the true aboral ring or annular plexus. It is 
more or less filled up with the brown cellular elements 
already mentioned as occurring in the central plexus, the 
dorsal end of which joins it (fig. 2, a. 6.). The space around 
it is no part of the true blood-vascular system, as supposed 
by Tiedemann and Greeff, but the perihemal canal corres- 
ponding to the aboral blood-vascular ring. This of course 
explains the injection of the tubular space from Tiedemann’s 
ring, as it is merely the perihemal canal of the central 
plexus which joins the true aboral ring, but does not terminate 
in it; for it passes upwards and attaches itself to the under 
surface of the disc just outside the madreporite (fig. 2, 2). 
Just in the same manner the structures hitherto described 
as the two gastric vessels are merely the perihemal canals 
corresponding to the real vessels which they enclose 
(fig. 2, p. b.). The ultimate ramifications of the latter are 
not known, but they contain the same brown cells as the 
rest of the blood-vascular system. 
Up to the time of Ludwig, the true vessels of the genera- 
tive organs had never been properly observed ; the structures 
described under that name by Tiedemann and others being 
merely their perihemal canals. They are ten in number, 
arising from the aboral ring (fig. 2, g. v.), and each expand- 
ing into a sinus around one of the spreading genital glands 
(fig. 8,g.). The perihemal canals enclosing them, which 
start from Tiedemann’s ring, are directly connected with 
the lacunar system in the body wall.- Hoffmann was led 
to suppose that in the Starfishes devoid of the interbrachial 
genital eat described by Miller and Troschel, the 
perihemal canals are in immediate connection with the 
internal cavities of the glands, the blood having direct 
access to the follicles. He imagined these canals to serve 
as ducts, the ova passing along them into Tiedemann’s 
ring, thence into the tubular space and out to the exterior 
