MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS. 173 
demonstrated, viz. the connection of this ring with a large 
plexiform bundle of vessels (figs. 2, 7, 8, c.p.), which ascends 
by the side of the stone-canal (s.c.) and joins an aboral 
blood-vascular ring (fig. 2, @..). This organ was disco- 
vered by Spix, and described by Tiedemann as a heart or 
heart-like canal, by Hoffmann as a glandular body, by Greeff 
as a gill-like organ, and then again as a heart by Teuscher 
and Ludwig, though the latter author has since abandoned 
the use of this term for the Echinoderms generally, and now 
speaks only of the central plexus. The organ in question 
is enclosed, together with the stone-canal, in a large tubular 
space, which is supported by one of the interradial falciform 
bands (figs. 7 and 8, a. p.). A person standing in the dorso- 
ventral axis of the Starfish with his feet in the oral ring, 
and facing the stone-canal, would see the central plexus to 
the right of it. It consists (21, 33) of a close network of 
vessels, partly dividing and partly anastomosing, the walls 
of which contain connective-tissue fibres, and perhaps 
muscle fibres also. The lumina of the vessels of the central 
plexus, and likewise of those in the oral and aboral rings 
connected with it, are filled up by large, brownish, cellular 
bodies of a peculiar character, which are also described as 
occurring in the celom and in the water-vascular system. 
Tiedemann described the central plexus as responding to 
stimulation by feeble contractions, and Hoffmann observed it 
contracting rhythmically, the same peculiarity characterising 
the two smaller plexiform bundles (fig. 2, p. 6.) which pro- 
ceed to the stomach from the aboral ring at the point where 
the central plexus joins it. 
The central plexus, like the rest of the blood-vascular 
system, is surrounded by a perihemal canal, which is the 
tubular space mentioned above as enclosing the central 
plexus together with the stone-canal (figs. 7, 8, a. p.). It 
may be termed the axial perihzmal canal. Its central end 
has been shown by Teuscher and Ludwig to arise from the 
inner perihemal ring-canal (figs. 5, 6,8, ¢.p.). This is 
only separated by a perforated septum (s) from the outer 
perihzemal ring (0. p.) which unites the perihemal canals of 
all the rays. 
The so-called “heart” was described by Tiedemann as 
terminating dorsally in an aboral vascular ring, from which 
proceed (1) ten genital vessels, (2) ten vessels to the pyloric 
ceca, (3) two gastric vessels, the plexiform bundles men- 
tioned above. These results were confirmed by Greeff and 
Hoffmann, except as regards the supposed vessels of group 
(2). These had been previously shown by the late Professor 
