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68 A N A T O M Y OF THE ECHINUS MARINUS.
lafteals pour the chyle into that veiTel in the mefentery which is the largeft, and
refembles onr vena cava.
A large roe, which almoit furrounds the redum, and is divided into five
lobes, difcharges itfelf by as many du6ls, which pierce the ihell near to the
anus, after communicating with each other (i).
T h e roe, with the inteftinal tube, are the chief parts which prefent within
die llicll; and to which that part of the ftrudure, which is by far the moft interefting
to the phyiiologift, and which I am about to defcribe, may be confidercd
as fubfervient.
BETWEEN the mner fide of the lliell and tlie parts I have been defcribing, to
wit, the inteiHnal tube and the roe, a large quantity of a watery liquor is lodged,
which taftes like fea-water, and contains a thirty-fecond part of its weight
of faline matter; which, chemically examined, was found to confift chiefly of
common or fea fait, but with all the other principles which fea-water contains.
This watery liquor is fecreted from the fea-water by means of the following
very beautiful ftru€turc.
T h e iliell of the echinus is pierced with upwards of four thoufand holes, difpofed
in five pairs of rows or phalanges, which extend from near the outer fides
of the teeth to near the anus.
T h e holes are difpofed on the outer fide of the ihell in pairs («) and with
each pair an abforbent velTel correfponds (*).
This abforbent vefi"el, in its collapfed ftate after the death of the animal, is
upwards of half an inch in length. Its end is covered by a flat plate; in the
middle of which is a hole, vifible to the naked eye, being about one hundred
and twentieth part of an inch in diameter, and very like one of our punila lachrymalia.
From the outer edge of thia plate a number of teeth projeft, like the teeth on
the wheel of a watch. "
T h e flat plate is very tough; contains fome cretaceous particles; and, when
preffed between the fore teeth, feels almoft like a plate of talc. It feems to
confift: of a number of pieces joined together; which are broader near its outer
edge than at the hole in its centre (_)!).
T h e duct from this plate to the iliell is compofed of pale-coloured circular or
tranfverfe fibres in fafciculi or bundles ; and two fmall bands of fach palecoloured
longitudinal fibres are obfervable on oppofite fides of the tube (z): or
its ftradure refembles our colon with the mufcular bands called its ligaments.
Thefe fibres, which have the appearance and the adlion, as we ihall find, of
mufcular fibres, are lined with a membrane.
When we trace the two holes which pierce the fhell, we find they diverge
(0 SceTab.XUIl.fig. 2. XYZ.
(«) Tab. XLIV. 8g. I. J.
W) See Tab. XLIV. Cg. 6. 7. 8.
(x) See Tab. XLIV, fig. 3. and 5.
(z) Tab. XLIV. fig. II. and u.
i S t : ;
A N A T O M Y or THE KCHINUS MARINUS. 6 J
ro oppofite fides of the row or phalanx of holes (a), and lead to leaves or
doubled membranes, not unlike the procefl!es or fubdivifions of the gills of a
toe
When I injeitcd qulckfilver into the mouths of the external abforbent vciTcls,
I found that it filled and diilended completely the internal leaves or doubled
membranes (i).
When, after this injection, I applied a common magnifying glafs, I could
diftin£tly obferve the du£ts by which the quickfilver entered the doubled membrane
: nay, I could trace a plexus of communicating branches formed upon it,
furrounded by a circular veiTcl, from which the quickfilver was conveyed by a
fingle tube into a large pipe, nearly one-twentieth part of an inch in diameter;
and which ferves as a trunk, receiving the fluid from one of the pairs of rows
or phalanges (</).
Each leaf or doubled membrane receives at leafl: two branches from different
external abforbents. Hence we fee the reafon why each external abforbent divides
into two diverging branches; that although an accident fliould deftroy one
of the external tubes, the internal membrane may not be without its fupply of
liquor.
The trunk lail defcribed, and there are five fuch (e), divides Into two branches,
which terminate in large facs or receptacles (y), over the fockets of the
teetli, communicating with each other: and from thefe the liquor paiTes down,
the fockets of the teetli, and is difcharged into the fea, on each fide of the
tooth, between the focket of the tooth and beginning of the cefophagus (i").
The external abforbent veiTel has not only the appearance of being mufcular,
bat contrads fuddenly when touched with fea-falt; and, like an earth-worm,
or the probofcis of an elephant, it poflefl'es motion in all diredions (¿); and
particularly the animal polTefi^es the power of flretching it to the length of an
inch and a half, and upwards.
When it is elongated, it bccomes much fmaller; and the plate at its end,
which, as I have obferved> appears flat after death, is puflied out into a conical
form, and the hole in its centre becomes much fmaller («).
The internal double membrane, with its vafcular plexus, is likewife evidently
mufcular, altering its fhape and fituation when it is touched rudely with a knife
or probe, or when fea-falt is fprinkled on it.
There arc no valves within thefe veiTels: for, from the internal trunks, the
plexus upon the doubled membranes, and the external abforbents, may be filled
with injedion; or when, after deatli, the teeth with their fockets are prciTcd
inwards, the watery liquor contained in the internal du£ts pafles outwards
tlirough the flicll, and fills the externaJ veiTels.
S No
(») See T.ib. XLIV. fig. I.i.5.
(c) Sec Tab. XLIV. fig. ij. D E.
(0 S« Tab. XLIII. fig. I. and 2. R S T UV.
(£) SccTab.XLIV. fig. J7.EE.
(0 See Tab, XLIV. fig. It, aadiJ,
(i) Tab. XLrV, fig. 13, D E.
{J) Tab, XL IVfig. 13. D E F, fig. J4. fig. 15,
(/) Tab.XLIV. GGGG.
Tab, XLIV.fig.9. lo.li. IJ..
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