236 ASCIDIE. 
their branchiz, that the tentacles may seem to be outside, and 
that this character may be taken in distinguishing the species.” 
The branchial cavity is a large sac flattened at its sides, 
varying much in diameter, depth and form, in the different 
species. 
However different the form and disposition of the sac, the 
internal tissue is always the same: it consists of an infinity of 
little vessels, which cross each other at right angles in quadran- 
gular masses. When viewed attentively, we perceive small 
vessels coming from the transverse ones, which unite by two 
extremities to the great branches, which are vertical, each of 
which pass one of the sides, or at least the edges of the sac : 
we must therefore conclude that one is the branchial vein, the 
other the branchial artery ; one of these is near to the heart : 
the valvules are perceived with difficulty ; and doubtless the 
blood does not pass through this branch of the heart: the 
opposite trunk is sometimes double, and receives the veins of 
the body: in fact, there are many vascular filaments ; but 
whether they go to the general tunic of the body or the gene- 
ral tunic of the viscera, is extremely doubtful. 
The left ventricle is aortic, but has no auricle at the reunion 
of the vena cava with the pulmonary artery. This ventricle or 
aortic heart is not always easily to be seen. 
The branchize are simply oblong, are situated towards the 
bottom of the sac, and are consequently equally as long as the 
body: but when the branchiz are shorter than the body, they 
are found in the middle of the tunic: the branchie are re- 
curved, and are always situated in the middle of the tunic of 
the body. 
In general the position of the heart is determined by the po- 
sition of the mouth, and more especially by that of the rectum : 
its form is oblong, narrower at each extremity ; its substance 
is extremely thin and transparent ; so much so, that it is some- 
times difficult to distinguish it in the middle of its pericardium: 
at one extremity it receives the trunk of the branchial veins ; 
on the other, the aorta, which distributes blood all over the 
body. 
