176 P, HERBERT CARPENTER. 
epithelium. On the other hand, the epithelial cells lining 
the stone-canal are high and ciliated, and its walls more or 
less plicated, in some species very much so; the plications being 
supported by calcareous rings of various forms. Towards 
its ventral end all the plication ceases and it joins the 
water-vascular ring as a simple tube. So far as we know 
with certainty, the water-vascular and blood-vascular 
systems are entirely distinct, though the injections of Hoff- 
mann and Greeff have led them to believe in a communi- 
cation between the two systems in the region of the disc. 
At any rate the old view, which was based chiefly on the 
results of injections, is no longer tenable, viz. that the two 
are connected by the ten “ brown bodies of Tiedemann,” 
small eminences resting on the water-vascular ring (fig. 6,7). 
Ludwig finds these structures to be lateral diverticula of 
the water-vascular ring, which are lined by an epithelium 
of cuboid cells, and contain the brown cellular bodies that 
have been described as present in the blood-vascular system. 
But there is no connection between these diverticula and 
either the perihzmal ring- canal (fig. 6, @. p., 0. p.) or the true 
oral blood-vascular ring {o. d.). 
The origin of the lateral trunks from the radial water- 
vessels is protected by a valvular arrangement discovered by 
Jourdain, the effect of which is to prevent a reflux into the 
radial vessels after contraction of the ambulacral vesicles for 
the purpose of expanding their corresponding tube feet. 
Lange and Ludwig have found this structure to be univer- 
sally distributed in the Asterids, and Ludwig describes it 
as also present in Ophiurids, and in some Urchins (16, 17, 
21, 27). 
Although Sars’ observations had led him to believe that 
Brisingais entirely devoid of a special blood- vascular system 
distinct from the celom, Ludwig has shown that in this as in 
other structural characters it is a true Asterid, having a 
dorsal and ventral ring and a central plexus connecting 
them. This organ was known to Sars, together with the 
perihemal space around it, and also the oral perihemal 
ring with its radial prolongations, but he did not regard 
the latter as in any proper sense entitled to be called blood- 
vessels, while he altogether denied the existence of any aboral 
blood-vascular system. 
2. Ophiuroidea. 
The minute anatomy of the Ophiurids has been inves- 
tigated by Lange, Teuscher, and Simroth, and most recently 
