66 A N A T O M Y OF THE ECHINUS MARINUS.
C H A P T E R XI I I .
A N A T O M Y of the ECHINUS MARINUS.
IN treating of the abforbent veflels of the ikate fiili, I obferved that the firft
origin, by open mouths, of feme of its cutaneous abforbents, could be demonftrated
by injecting liquors from the trunks into the fmaller branches of
certain lymphatic veffels; and I mentioned abforbent veflels pVoper to one of
the clafs of worms, to wit, the Sea-egg or Echinus efculentus of Linnxus,
G. 299. S. I. which I had difcovered and read an account of to the Philofophical
Society of this place in Auguft 1761. I lhall therefore add, as an Appendix to
this work, the defcription of this very curious animal.
I was led to examine the ftru£lure of it, by having been appointed by the
Philofophical Society of Edinburgh to report to them my opinion of a paper on
the fubjedt, which had been prefented to the Society in November 1760 by the
late ingenious Dr Thomas Simfon fenior of St Andrew's.
The-Dodor, in his paper, which was afterwards withdrawn, dwelt chiefly on
the want of brain in this animal; from which he thought additional force was
given to the doctrines he had publiihed in his EiTays on the Vital and Involuntary
Motions.
I entitled my Paper, Further OBSERVATIONS on the SEA-EGG or ECHINUS.
Since that time, I have traced with greafer accuracy fome particulars in the
ilracture of its parts; which, though few in number, are fo- lingular and remarkable,
that the knowledge of them ferves to enlarge our views in interefting
points of phyfiology.
THE ihcll of the Echinus is covered with a Ikin, and has many thoufand
thorns articulated with it by means of mufcular ligaments (I): Hence the thorns
fervc in placc of feet; and are fo tenacious of their powers, that I have feen
the pieccs of a broken iliell walk off in diiFerent dire£tions. Yet there is no
appearance of any organ like to the brain.
It does not, however, follow that they are deftitute of nerves; fmce thefc
may exiil independent of the brain, and be fo fmall as to efcape obfervation.
In the interfticcs of the thorns there are three kinds of bodies, foft at tlic
ends, fuppoitcd on calcarcous ilalks inclofed in a membrane, and articulated
with
(/) See Tab. XLIJI. acd T:ib. XI,IV. %. 3.
A N A T O M Y OF THE ECHINUS MARINUS.
with the iliell by means of mufcular membranes. Not only the roots but the
points of thefe bodies, which are fliorter than the thorns, are in continual motion,
poíTeíIing the powers of opening and lliutting hke the fingers of our hand,
and having thefc points fupported by a mixture of cretaceous particles with
mufcular fubftance (?«).
Thcfe bodies fomewhat refemble the antennae of infecís, and probably fupply
the place of the organs of the fenfes in the more perfe£t animals.
The mouth is furniflicd with five teeth, with large fockets («), which arc
tied to the ihell by a very ftrong membrane; around which there is placed, on
the inner fide of the flicil, an irregular ftrong circle of cretaceous matter, from
which a pair of niufcles is extended to each tooth, and other mufcles join the
fockets of the teeth to each otlier ((?).
The cefophagus (p), after running about three inches in length, and being tied
by a ligament to the fide of- the anus, makes a turn, and foon opens into a •
wider canal or inteftine, nearly in the way our ilium opens into the fide of the
colon; and this inteftine, after defcribing two waving circles around the ihell,
defcends to the anus (j').
On the under edge, of the whole length of the mefentery, I found two veffels
without valves, nearly equal in fize and parallel to each other, which I injedted
with quickfilver (r); and from them filled a beautiful net-work of vclTels,
not only on the inteftines (J), but difperfed on fine membranes, which tie the
intefl:ine to the inner fide of the ihell.
I could not, however, obferve that thefe two veiTels communicated with each
. other directly, nor by the medium of any organ like to our heart; nor could I
.obferve in the living animal any beating organ like to the heart: yet near to the
anus, and connected to tlic rectum, which is the place of the heart in many
other worms, I found a fmall organ W , Tab. XLIII. fig. 2. which feems to be
hollow«
I t appears highly probable, that one of thefe veflels is the principal artery
or aorta, and the other analogous to our vena cava; and that they communicatc
by invifible branches, and circulate the blood by the mufcular action of their
coats, without the intervention of a heart, nearly in the way the vefiTels in fiilies
carry the blood from the gills back to their heart.
In what manner the chylous or lafteal veins communicate with thefe two
velTels, I have not yet been able to difcover.
In fome animals, as in the human taenia, or fafciola hepatica in ilieep, the
osfophagus, I have obferved, may be confidered as the aorta, conveying the
chyle or bile dire£tly to all the parts of their bodies; but here it is probable the
ladeals
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m I
'HKilti
(«}Tab. XLIV. fig. 18.19: 20. 21.
(c) SccTab.XLin. fig. i.D.E. Tab. XLIV. Cg. 13. 17.
(,) See Tab. XLIir. F G H IK L M N 0 P (i., »nd Tab. XLIV. G K L (i,
(r) SceTitb. XLHl. fig. I. KLMNOPQ^.
(«) See Tab. XLIV. fig. j.fig.ie.jy.
(/.) Tab. XLIV. fig. .7.
(/) Tab.XLni. M.
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