Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/40/
32 
ECHINODERMATA. 
median line, and two smaller ones (b, b,) 
placed laterally. On the sides of the my the 
calcareous substance is disposed, as it were, in 
ribs (c, c, fig. 9) ; these rise from the floor at 
first nearly parallel with each other, and are con¬ 
nected by cross bars, but on approaching the 
upper part or roof of the ray they cross in all 
directions and form an irregular network, the 
intervals of which are occupied by softer inte¬ 
gument. The ribs and bars are made up of 
small pieces joined by plane but oblique sur¬ 
faces, a mode of construction calculated to 
admit of their being lengthened and shortened 
upon one another, and thus to allow of the ca¬ 
vity they surround being dilated and contracted. 
Fig. 9. 
Portion of a ray of Asterias rubens viewed laterally. 
A broad calcareous disk is situated on the 
upper surface of the body, in the angle be¬ 
tween two of the rays, (figs. 12 andlô, z,) which 
is connected internally with a singular organ 
named by Tiedemann the sand canal, to be 
afterwards described. The calcareous pieces 
are of a homogeneous structure, without cells 
or fibres ; they consist, according to Hatchett’s 
analysis, of carbonate of lime, with a smaller 
proportion of phosphate of lime. 
The coriaceous membrane which connects 
the pieces of the skeleton is made up of white 
glistening fibres. It is contractile and irritable, 
for it slowly shrinks on being scratched with 
the point of a knife, or when it is cut through. 
The external membrane is much thinner and 
softer than that just described ; in various parts 
it is coloured, or in these parts there is a co¬ 
loured layer underneath it. 
The appendages or processes on the surface 
of the body are of three kinds. First, calcareous 
spines ; these are found over the whole surface 
except the grooves for the feet They are at¬ 
tached by a moveable joint at their base to the 
calcareous pieces of the skin, and aie invested 
by the external soft membrane nearly as far as 
their point. Those on the upper surface are 
solitary, short, and for the most part club- 
shaped, their broader summit being marked 
with radiating points ; whence they were named 
stelliform processes by Tiedemann. On each 
side of the groove for the feet the spines are 
thickly set (c, c, fig. 7); these inAstenas 
rubens form three rows, in the middle and 
innermost of which they are placed three deep. 
On this part of the surface they are also longer 
and pointed. The spines are slowly moved at 
the will of the animal. , . , . 
The appendages of the second kind are ot a 
very singular nature ; they have the appearance 
of pincers or crabs’ claws in miniature (fig. 298, 
c, b, b, p. 615, vol. i.) and were described 
by Miiller as parasitical animals under the 
name of Pedicellaria. Monro gave the name 
of antennae to analogous organs which are 
found on the sea-urchin. They probably do 
not exist in all species, for Tiedemann makes 
no mention of them in his description of A. 
auranliaca. In A. rubens they cover the 
surface generally, and form dense groups round 
the spines. Each consists of a soft stem 
bearing at its summit, or (when branched) at 
the point of each branch, a sort of forceps of 
calcareous matter not unlike a crab’s claw, 
except that the two blades are equal and similar. 
When the point of a fine needle is introduced 
between the blades, which are for the most 
part open in a fresh and vigorous specimen, 
theÿ instantly close and grasp it with consi¬ 
derable force. The particular use of these 
prehensile organs is not apparent ; their stem, 
it may be remarked, is quite impervious. 
The third sort of appendages consists of those 
which are named the respiratory tubes ; they 
will be considered afterwards. 
The other genera of Asteroidea have also a 
cutaneous skeleton presenting the same general 
mode of construction as that of Asterias, but 
with certain modifications of structure and still 
greater differences of form in particular cases. 
Of these we may here notice the crinoid echi- 
nodermata and the genus comatula, as the 
most interesting examples. The former ani¬ 
mals, comprehended by most naturalists in the 
genus Encrinus, are, with one exception (The 
Enc. caput medusa or Pentacrinite) found only 
in a fossil state, and the remains of their ske¬ 
letons constitute the fossils named encrinites, 
trochites, entrochites, &c. An idea of their 
structure may be obtained if we imagine an 
asterias placed with its mouth upwards on a 
columnar jointed stem, one end of which is 
connected to the dorsal surface of the animal, 
and the other most probably fixed at the bottom 
of the sea. The rays or arms extending from 
the circumference of the body are much 
branched, and at last pinnated ; other jointed 
processes, named auxiliary arms, surround the 
stem in whorls placed at short intervals. The 
column is perforated in its centre with a narrow 
canal, down which a prolongation of the sto¬ 
mach extends, and lateral canals proceed from 
the central one through the verticillate auxiliary 
arms. The Comatula has rays spreading from 
the circumference of the body, branched and 
pinnated like those of the pentacrinite. It is 
not fixed on a column, but the dorsal surface 
of the body is elevated in the middle, and 
bears a number of smaller rays or arms, and 
this dorsal eminence with its rays has been 
sometimes compared to a rudiment of the 
column of the pentacrinite with its auxiliary 
arms. Besides the mouth there is an anal 
opening on the ventral surface, situated on an 
eminence near the margin.* 
b, In the sea-urchin the calcareous matter is 
disposed in polygonal plates, which, being 
* Meckel, Vergl. Anat. ii. p. 31.
        

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