130 CLASS IY. 
Notwithstanding much careful investigation, there still exists 
great obscurity about the circulation of the blood im Echinoderms. 
TIEDEMANN and DELLE CHIAJE give very conflicting descriptions 
of the vascular system—the difference being founded in the inter- 
pretation of the dermal vessels, which are connected with the 
organs of motion. The first of these authors considers the motion 
of fluid observed in these vessels to be altogether distinct from the 
circulation, whilst, according to the other, they are a part of the 
system of blood-vessels. In Asterias TinpEMANN found on the 
inner surface of the skin of the back, a vascular ring, which he 
considers to be venous. The vessels which run upon the surface 
of the visceral appendages of the rays open into this ring. From 
it a canal arises, which performs the office of a heart, lying near 
the so-called lime-canal which is found there. The canal runs 
into a vascular circle surrounding the mouth, which TrepEMANN 
holds to be arterial, and from which branches proceed to the 
intestines. Besides these two vascular rings (one on the dorsal 
and one on the abdominal surface), there is a third ring of an 
orange-yellow colour found on the inferior surface beneath the skin. 
TIEDEMANN was not able to discover any communication between 
this ring and the rest of the vascular system. In Echinus vascular 
rings occur, in like manner, round the mouth and the anus, on each 
surface two, of which one is to be considered arterial, the’ other 
venous. The heart is oblong, divided into many cells, and lying 
on the cesophagus’. In Holothuria there is a circulating system 
without a heart, or rather the heart has the form of a contractile 
vessel, that runs at the outside on the surface of the intestine. 
At the anterior extremity of the intestinal canal this vessel forms a 
vascular circle, whence very fine branches arise; when near the 
anus it has become small, having given off a multitude of fine 
branches, which run on the surface of the intestine. There isa 
transverse vessel which connects the longitudinal trunk on the first 
loop of intestine with that on the second. Many intestinal veins, 
which seem at the same time to perform the part of absorbents or 
1 Comp. the descriptions and figures of VALENTIN, Anatomie du genre Echinus, 
pp. 897—96. Tab. vir. fig. 119, 125, 127. Tab. vim. fig. 144-152, &c. There isa 
figure also of the heart and part of the blood-vessels in Spatangus in CuviER’s Régne 
Anim. éd. illustrée, Zoophytes, Pl. 11 bis. 
i 
