MOLLUSCA. 369 
ed through the meshes of such a membrane, and the food Mollusea. 
Mollusca. at the sides, and has its axis vertical ; its. cavity terminating L 
to be carried along it by invisible villi; but this is mere << 
ee upwards in the oval opening, and being closed at the bot- 
tom. It is united to the envelope, er to the mantle above 
and behind; the juncture, beginning in front of the oval 
opening, extends backwards on each side of it, and then 
downwards in two lines; between these, along the middle 
of the back, is a vertical compound stripe, that seemed to 
me cartilaginous. At the bottom the sac appears to be en- 
veloped by the soft substance of the mantle, but at its sides 
and front a vacant space is left between them, that ends in 
the opening of the funnel. The branchial sacis more com- 
presed towards its lower part; and here are placed, exter- 
nally to it, the heart on the left, and the stomach and other 
viscera on the right side, the vent opening upwards at the 
front into the funnel. On its.sides and front the sacis per- 
forated by four rows of narrow, vertical, irregularly oval 
holes or spiracles, about sixteen in each row, placed-at less 
than the diameter of one apart from each other. Through 
these the water, which flows constantly in at the mouth 
when its orifice is open, appears to be conveyed to the va- 
cant space between the sac and mantle, and it then escapes 
at the funnel. The sac seems extremely thin between the 
spiracles ; but their edges are thickened, as if cartilaginous ; 
and they are lined with closely set ciliae, which, by their 
motion, cause the current of water. When these are in 
full activity, the effect upon the eye is that of delicately- 
toothed oval wheels revolving continually, in a direction 
ascending on the right and descending on the left of each 
oval, as viewed from without; but the ciliae themselves are 
very much closer than the apparent teeth, and the illusion 
seems to be caused by a fanning motion given to them in 
regular and quick succession, which will produce the ap- 
pearance of waves, and each wave here answers to a tooth. 
The spaces between the rows of spiraclesiare of much more 
substance than the intervals of the spiracles; some liga- 
ments are stretched from them across the side cavities to 
the mantle, which seem intended to keep the branchial sac 
expanded. These spaces also support finger-like processes, 
about eight in a row, that project nearly at right angles into 
the central cavity. 
The central cavity I shall venture to call the mouth, 
though the mouth is said by Cuvier to lie at its bettom. 
The large short tube at its opening ends in five or six ob- 
scure indentations ; it can be drawn in and closed at the 
will of the animal, as can the opening of the funnel. At the 
bottom of the tube the entrance of the mouth is guarded by 
simple tentacula, some longer, some shorter, ranged subal- 
ternately: their number was not ascertained. Whatever 
little substances, alive or inanimate, the current of water 
brings, flows in unless stopped by the tentacula, and they do 
not appear fastidious, to the mouth, and lodge somewhere 
on the sides of it. A lively animalcule will sometimes dis- 
engage himself by struggling, and dart about in the cavity 
till he lodges on some other part; or, if a morsel is found 
unsuitable, it is ejected by the funnel’s being closed, and 
the branchial sac suddenly contracted vertically. Mostly, 
however, whatever part the food lodges on, it travels from 
thence horizontally with a steady slow course towards the 
front of the cavity, where it reaches a downward stream of 
similar materials, and they proceed together, receiving ac- 
cessions from both sides, and enter at last at the bottom, the 
zsophagus : this is a small flattened tube which carries them, 
flowing on in the same way, without any effort of swallow- 
ing, towards the stomach. The tube takes a sharp curve 
upwards and backwards before arriving there. 
It is extraordinary that these particles pass along in the 
mouth just behind the spiracles, when the ciliae are in full 
activity, without being at all affected by them. I have, in 
some positions, seemed to catch a glimpse of a membrane 
suspended within, too transparent to be commonly seen. 
One may imagine the water to pass to the spiracles, strain- 
VOL. XY. 
conjecture. The projecting fingers have the effect, whether 
intended for such a purpose or not, of detaining some pri- 
soners more bulky than the usual food of the animal, for, in 
several individuals, I met with small shrimp-like crustacea 
confined. between the rows: one escaped during an obser- 
vation, another, after three days, seemed as lively as when 
first swallowed. 
The stomach runs backward horizontally ; its fore-part 
had an inflated look when seen from the side, and, when 
from below, that of possessing two lateral lobes. The food 
after accumulating here was observed to be pressed onward 
to the hinder portion, leaving a narrow opake line of con- 
nexion with the cesophagus; the rest of the fore-part, of 
which the apparent volume was nearly as before, having an 
ochreous tint ; this was inferred to be the liver, enveloping 
the stomach above and on the sides, and accords with its 
place in other ascidiae and mollusca. The line is continu- 
ed by the intestinal canal that rises and then bends forward, 
taking the form of a reversed §, and terminates in an as- 
cending rectum and sphincter. The feces are considerable, 
as might be expected, where the food is taken with:so little 
discrimination. Transparent vessels, that may be supposed 
lactaels, ramify along a part of the intestine and meet at a 
collection cf globular bodies, from whence two flattish lobes 
extend backward ; in others these are wanting. From the 
meeting of the vessels two branches ran, one downwards 
and backwards, which was lost under the stomach, the other 
forwards ; and from the direction it took, I suppose it might 
communicate with a main stream of blood near the heart. 
Some individuals had not the projection above the vent 
observable in others. 
But the part that struck me as most remarkable in this 
creature was the circulation, of which a good view can be 
obtained through the transparent coat, for the particles of the 
blood are numerous, and, though not uniform in size or shape, 
are mostly between :00025 and -0002 inch in diameter, and 
approaching to globular. They are easily measured, as in 
the intervals between the spiracles, they pass mostly but one 
at a time. 
The creeping tube, which unites the individuals of a 
group, is the channel for two separate currents of blood, an 
upward and a downward one, that are flowing at one and 
the same time, and that send off each branch to every pe= 
duncle : the blood thus passes into the animal by one cur- 
rent, while another carries it back. One of these canals 
communicates at the termination of the peduncle with the 
heart, which is placed, as has been mentioned, near the bot- 
tom of the branchial sac on the left side, and consists of a 
transparent ventricle, or boyan, running forward anda little 
slopping downward, in a channel hollowed to contain it. 
Along the whole length of the boyau a part on one side of 
its axis seems fixed to the channel, the rest free and con- 
tractile. 
When the blood entered the heart from the peduncle, 
contraction began at the middle of the ventricle, impelling 
onward the contents of the fore part ; and the contraction 
of the back part followed in the same direction, so as for the 
whole to have the effect of one pulsation ; the heart was 
then filled again by a flow from the peduncle. The inter- 
vals of the pulse were pretty regular in the same individual, 
but in different ones they varied from two seconds to one 
and a half second. Part of the blood thus impelled formed 
a main upward stream along the front of the branchial or- 
gan, branching off at each of the horizontal passages between 
the rows of spiracles, and at one above them on a line with 
the junction to the mantle on each side. All these again 
united and formed a downward current behind. The hori- 
zontal channels were connected also by the smaller vertical 
passages between the spiracles; the set of the ee in 
A 
