210 SUPPLEMENT 
pretty nearly all of them the same mode of respiration. ‘The 
labial appendages with which the mouth is provided appear, 
by their continual movements, to produce a sort of current in 
the water where the animal is plunged. This is very well dis- 
tinguished, especially in the species in which the posterior 
extremity of the mantle is prolonged into two tubes, more or 
less long; the water enters by the lower, and issues out by 
the upper. Itis thus in the ascidiz, and perhaps also in the 
biphore. It is at the time of the passage of the fluid into the 
branchial cavity that the effects of respiration take place. 
In all the malacozoaria the circulating system is complete, 
although the re-entering or absorbing centripetal system is 
only formed by veins. 
These vessels have their parietes extremely thin, and often 
so much confounded with the tissue of the parts, especially in 
the dermoid envelopes, that it is often rather difficult to per- 
ceive them; and sometimes they really exist only through the 
internal membrane, as in the bivalves. ‘They sometimes pre- 
sent this peculiarity, that they are pierced with gaping orifices, 
tolerably large, at least in the visceral cavity, as may be well 
observed in the aplysiz. We also find that they sometimes 
bristle with sorts of little spongy bodies, also proceeding into 
the visceral cavity. 
As in the animals of more elevated types, the veins spring, 
without doubt, in part, from the continuity of the arteries, and 
in part from the tissue itself of the organs, but they never con- 
stitute more than two systems, one which returns from the 
whole body, and the other from the special organ of respira- 
tion; that is to say, there is no system of the vena porte. The 
venous radicles of the general system of the body, after having 
been successively united in trunks more and more thick, dis- 
tinguished, however, for some time into those of the viscera, and 
those of the sensible contractile envelope, arrive towards the 
respiratory organ, and according as it is simple or complex, 
a 
