THE STRUCTURE AND AFFINITIES OF THE SEA-SQUIRTS. 243 
through the vascular sieve, passes out through the stigmata ’’ 
into the ‘‘ atrium,” and leaves the body of the animal by the 
exhalent orifice. The circulation in these animals is indeed — 
to borrow an expressive v/ord from G-oethe — a “ Zauberfluss,’* 
for instead of a definite onward current there is actually a 
tidal ebb and flow. 
The heart is a muscular tube of considerable length, open at 
either end, but having no valves. It lies in the region of the 
lower border of the stomach, between the mantle and lining 
membrane, and is invested with a fold of the la|ter, in the fashion 
of a pericardium. 
The arrangement of the vessels has been most carefully studied 
and clearly described by Mr. Hancock, but is somewhat com- 
plicated; so that, were there sufficient space iiere for descrip- 
tion, our readers would weary. Suffice it to say that there are 
two main longitudinal trunks, which eventually terminate at 
either end of the heart ; one of which runs along the endostyle, 
while the other courses along the opposite side of the gill-sac. 
These are brought into relation by means of the transverse 
vessels of the gill-sac, and by a circular channel situated just 
below the anterior collar.” 
Owing to the connection of the main trunks, either imme- 
diately or mediately, with certain networks which ramify in the 
mantle and over the digestive tract, and with vessels which 
serve the test, the blood which is returned to the heart by either 
of these channels in its turn, arrives in only a partially aerated 
condition ; that trunk, however, which courses along the endo- 
,style being the carrier of the least pure fluid. 
The number of heart-pulses in either direction is not constant, 
but varies considerably. As regards their duration, Mr. Han- 
cock found that it “ required 2-j- minutes to accomplish the beats 
during a single oscillation.” 
The digestive system is comparatively simple. The mouth, 
which is situated at the bottom of the gill-sac, leads almost 
directly into the stomach, upon the floor of which there is a 
longitudinal fold, which is continued into the intestine. The 
tube fo^ed by this latter and the stomach is folded twice upon 
itself, the concavity of the first loop looking toward the heart 
(haemal flexure, Huxley). In the second loop the reproductive 
organs usually lie. The anus opens into the atrium, in the 
neighbourhood of the exhalent orifice (fig. 10, m, st, i, and a). 
The food, which is sedimentary, consisting of Diatoms, &c., is 
not selected by the animal. It is sifted from the water which 
traverses the gill-sac by ciliary action, accumulated at the “ oral 
lamina,” before mentioned, and conducted along this organ, 
formed into a cord by a mucous secretion, to the mouth. . 
The liver is a gland made up of delicate branching tubes which 
