8 M. Z: Gerbe on the Vascular and Nervous Apparatus 
sac, varying in form according to the species, with internal mus- 
cular columns and delicate transparent walls, formed of longi- 
tudinal and annular muscular fibres imtercrossed in various 
directions; it presents on each side a single semilunar fissure, 
to which a valve of the same form is adapted internally. From 
this contractile sac issue all the arteries which perform the dis- 
tribution of the blood. 
The second sac, which is much larger and has its walls 
thinner and less muscular, completely envelopes the arterial 
heart, and communicates by two or three oblong apertures with 
the same number of large venous lacune, which convey the 
blood to the heart. This enveloping portion of the central cireu- 
latory organ has been assimilated to the pericardium of the red- 
blooded animals. In this assimilation there is an appearance of 
truth, if we consider only the form; but it is far from being 
exact if we take into account the function, which is very different 
in importance from the form. ‘The pericardium in the Verte- 
brata is an organ closed everywhere, without any communica- 
tion either with the cavities of the heart or with the vessels 
which run to it; here, on the contrary, the sac which has been 
assimilated to the pericardium directly receives all the blood 
into its cavity and transfers it to the ventricle. It is inter- 
mediate between the venous lacune and the arterial heart, and 
fulfils exactly the part which, under another form, the auricle, 
in fishes for example, performs with relation to the vene cave 
and the ventricle. By its functions, therefore, this second 
cavity would be the analogue of the auricular portion of the 
heart in Vertebrata. 
Five arterial branches issue from the anterior extremity or 
half of the central contractile sac; only one springs from its 
posterior extremity. Of the five anterior arteries, one (the 
ophthalmic artery) follows the median line, passes directly to 
the brain, and is distributed in the ocular peduncles. In those 
species in which the rostrum m the young state acquires the 
form of a long spine, the ophthalmic artery is produced to the 
extremity of this appendage, after having furnished a branch 
to each eye. This arterial branch, which is one of the largest, 
is furnished at its issue from the heart with a double valve, or 
rather with two opposite flaps, separated at the base, in contact 
at the apex, which alternately open and close to let pass the 
globules of the blood and prevent their flowing back into the 
heart. The action of these flaps, which is completely imde- 
pendent of the contractions of the central organ, is sometimes 
slow, sometimes rapid; frequently it is even suddenly and 
momentarily suspended. ‘Two other branches, one on each 
side, originating a little behind the preceding, also run forward, 
