? 0 A N A T O M Y OP THE ECHINUS MARINUS. A N A T O M Y O.F THE ECHINUS MARINUS.
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No communication of. the internal du£ts and plexus with the ca\'ity withhi
the iliell is difcoverable by the injeclion of quickfilver.
On reviewing the ftru£ture of thefe duds, there can be no doubt that the
fea-water is abforbed by the external open-mouthed veiTels, and conveyed from
them through the iliell into the plexus of the internal doubled membranes: from
which a fccrction of part of it is made by invifible veiTels into the cavity of the
ilicll, while the remainder of the fea-water paiTes from thefe plexus into the five
large internal duils ; and ,from them through the receptacles at the roots of the
fockets of the teeth, to be difcharged into the fea b y ten apertures at the fides of
the teeth.
We muft likewife fuppofe that there is a conftant change of the water within
the iliell; and that there are therefore abforbing veifels which correfpond with
the fecerning. Thefe are invifible ; and whether they terminate in tlae plexus
and five inner du£ts, or with the lacleals in the circulating fyflem of the animal,
may perhaps be a queftion.'
No other individual of the animal kingdom feems to afford fuch an opportunity
of inveiligating the ilruclure of an abforbent vciTel, and of obfei-ving how it
performs its office.
When the external abforbent is elongated by the a£tion of its mufcular fibres,
it becomes fmaller in proportion to its elongation; and it likewife acquires
a tenfion or degree of rigidity. The jointed plate wliich covers its beginning,
is at the fame time thruft out into a conical form; and the diameter
of the hole or perforation in its middle, inftead of becoming larger, is fo very
much diminiilied, as fcarcely t o be diilinguifhable with a magnifying glafs of
two inches focus.
While the fuBe is: elongated, and while the plate at its end preferves its conical
figure, I have never been able to "obferve any motion of the fides of the
hole refembling the motion of the Hps or mouth of an animal.
As the tubes are thick coated, and the fea-water has little colour, I could not
perceive it entering the tubes, or moving within them, fo as to be able, from
ocular obfervation, to determine the motions the tubes perform at the time they
abforb.
In a few experiments, 1 coloured the fea-water with milk, with indigo, and
with madder; but have not yet fecn thefe colours enter the abforbents. I am,
however, far f rom defpairing of fuccefs in fuch experiments.
UPON the whole. There feems to belittle or no reafon for fuppofing, that the
fides of the hole in the plate, at the origin of the abforbent tubes, open and
•ihut the orifice alternately; or that fuch a motion takes place at our punda
lachiymalia, or at the mouths of the abforbent lymphatic veflcls I have defcribed
on tlie Ikin of the fkate; or that the reception of hquors into thefe veflels,
or in general into the abforbent veflels of animals, depends on an a£kion like to
that of the lips and mouth of animals in eating.
Wc
We are, on the contraiy, from the fa£ts above ftated, led to conclufionj
that the abforbent tube is, by its proper mufcular adion, and perhaps alfo by
an influx of liquors into the arteries which are difperfed on its coats, made
tcnfc, and its cavity at the fame time much contraded: in confequence of
which it acquires the property of a glafs capillary tube, of attrading liquors;
which, by the well-regulated adion of its mufcular fibres, are puihed onward
from their entrance to their termination.
Thefe abforbent tubes ferve for the farther purpofe, which at firil fight we
fliould not have expeded, of feizing and fecuring the buccina, on which the
animal preys.
I was led to fufped this from obferving, that when I put them into a glafsveflel
with water, in order to view the adion of- their abforbent veflels, they
fixed themfelves by their abforbent tubes to the fides of the glafs. •
It immediately occurred to me, that, by a fimilar adion, they might fecure
the ilielled worms, on which I knew they preyed, as I had found particles of
fliells in their alimentary canal.
I therefore direded the fifliermen to bring me, along with the echini, fome
living buccina: to which, as I had fuppofed they would do, they attached themfelves
fo effedually, that when I lifted the echinus out of the water, I found it
could fupport with cafe a buccinum which weighed nearly a quarter of a
pound.
Perhaps, for this particular purpofe, the plate at the mouth of the abforbent,
which they apply as a boy does a piece of wetted leather to a ilone, is fo conftruded
as to^be made flat; while, for the abforption of liquors, it is puihed
out into a conical fiiape, and has its orifice contraded.
E X P L A N A T I O N
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