Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea
Todd, Robert Bentley
Fig. 326. 
within these sacs, as upon the gills or lungs of 
other animals, but the exact course of the blood 
does not appear as yet to have been satisfacto¬ 
rily ascertained in these animals. Audouin* 
believes it to be essentially the same as in the 
Crustacea. The long-shaped dorsal vessel or 
heart gives off arteries to both sides, and re¬ 
ceives at one place branches from the gills. 
The veins form only spaces or sinuses, and not 
vessels on the abdominal side of the animal. 
The blood propelled from the artery is passed 
through the system, returning from which, it is 
collected into the venous sinuses below, thence 
it proceeds to the pulmonary organs, and after 
passing through them, returns to the heart. 
Zoophytes. — The general character of the 
circulation in this class is exceedingly ob¬ 
scure ; for while in some of the animals be¬ 
longing to it, comparative anatomists have not 
succeeded as yet in pointing out any distinct 
vascular system ; in others, they have been at a 
loss to determine, among various vascular or¬ 
gans, which.of them forms the proper circula¬ 
tory system corresponding with that of higher 
Echinodermata.—Among the Zoophytes the 
Echinodermata present the most fully deve¬ 
loped vascular system with which we are ac¬ 
quainted. According to the observations of 
Tiedemann and Delle Chiaje, who have inves¬ 
tigated the structure of these animals with great 
success, there are two principal divisions of the 
vascular system, described by the first of the 
above-mentioned authors as distinct from one 
another, by the other as communicating toge¬ 
We do not feel inclined to consider, in ac¬ 
cordance with the view of these authors, that 
series of cavities which is employed in loco¬ 
motion as a part of the nutritive circulatory 
That part of the vascular system of these 
animals again, which is situated in the neigh¬ 
bourhood of the alimentary canal, very proba¬ 
bly corresponds with the circulatory organs 
which we have been describing in other ani¬ 
mals ; since arteries and veins can be distin¬ 
guished in it, and there is good reason to be¬ 
lieve that a circulation of fluid takes place 
through its vessels in all the kinds of Echino- 
dermatous animals. 
In the Holothuria, the principal artery or 
heart is connected with a ring situated round 
the commencement of the alimentary canal, 
from which the systemic arteries are given off: 
the systemic veins send branches to the gills, 
and the returning vessels from these organs 
transmit the circulating fluid through one large 
trunk into the heart. 
The intestinal vascular system of the Asterias 
and Echinus is somewhat similar to that of the 
Holothuria, consisting of annular vessels, from 
which arteries and veins are given off, and con¬ 
nected with a dilated contractile canal, consi¬ 
dered as a heart. 
Planaria.—Next to the Echinodermata in 
respect of the degree of perfection of their cir¬ 
culatory organs, may be mentioned the Plana- 
riæ, in which M. Duges* has pointed out a 
very remarkable system of vessels which ap¬ 
pear to constitute circulatory organs (fig. 327, 
a, a). For some time previously to the disco¬ 
very of these vessels, the sin¬ 
gularly branched intestinal ca¬ 
vity of the Planaria and some 
Entozoa was believed to hold 
the place of organs of circula¬ 
tion, the same cavity in which 
digestion occurs being believed 
to carry by its ramifications the 
nutritious fluids to different 
parts of the body. But Duges 
has shewn the existence in them 
of a system of vascular organs 
resembling considerably those 
of the Leech, to which animals 
the Planaria bears, in other parts 
of its organization also, astriking 
analogy. The vascular system 
of the Planaria consists of three 
principal longitudinal trunks, 
two lateral and one dorsal or median, which are 
all united together by numerous minute anasto- 
Fig. 327. 
* See the article Arachnida, p. 206. * Annal, des Sciences Natur, xv. p. 160.


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