Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/579/
SHELL. 
569 
which rises up in ridges upon the exterior. 
Hence it would appear that, like endogenous 
trees, whatever additions these spines may 
receive in length, they can receive little or 
none in diameter. The slender, almost fila¬ 
mentary species of the Spatangaceæ, and the 
innumerable minute hair-like processes at¬ 
tached to the shell of the Clypeasteridæ, are 
composed of a like regular reticulated tissue ; 
many of these are extremely beautiful objects 
when examined with the microscope without 
any preparation. It is interesting also to 
remark, that the same structure presents itself 
in the Pedicellarice, which are found upon 
the surface of many Echinida, and which have 
been so great a source of perplexity to 
naturalists. The complete conformity which 
exists between the structure of their skeleton, 
and that of the animal to which they are 
attached, would seem to remove all reasonable 
doubt that they are truly appendages to it ; 
as their actions also would indicate. 
The same structure presents itself in the 
calcareous plates which form the less perfect 
skeletons of the Asteriadce, and also in their 
spines, when these (as in the large Goniaster 
equestris) are furnished with a calcareous 
frame-work, and are not mere projections 
of the hard integument. It is also met with 
in the family Ophiuridce, which forms, in 
some respects, the transition to the Crinoidal 
group ; but the calcareous skeleton is here 
generally subordinate to the firm and almost 
horny integument. In the Crinoidea, on the 
other hand, the calcareous skeleton is highly 
developed, and its structure is extremely 
characteristic. This is well displayed in the 
recent Pentacnnus Caput Medusœ, the stem 
and branches of which are made up of a 
calcareous net-work, closely resembling that 
of the shell of the Echinus. There is ex¬ 
hibited, moreover, in a transverse section 
of the stem of Pentacnnus, as in the spines of 
Echinus, a certain regular pattern, which 
results from the varying dimensions of the 
areolae in different parts. This pattern, 
formed by the extension of five ‘pairs of rays 
(strongly reminding us of the medullary rays 
of plants) from the centre towards the cir¬ 
cumference, is frequently well preserved in 
the fossilized stems of Pentacrini, and varies 
in different species sufficiently to serve as 
a distinctive character. In the round-stemmed 
Encrinites, a transverse section of the joints 
exhibits a simple concentric arrangement. 
It only remains for us to notice the order 
Holothuridœ, in which, as is well known, the 
calcareous skeleton of the other Echinoder- 
mata is reduced to its most rudimentary con¬ 
dition; never forming a complete and con¬ 
nected framework, but only showing itself in 
detached pieces, the disposition of wrhich 
is extremely variable. In the typical Holo- 
thuria, there are five solid calcareous plates 
around the mouth, in which the calcareous 
reticulation is very characteristically seen. 
Each of the tentacula, also, has a small cal¬ 
careous disk at its extremity, which presents 
a sort of rude sketch of the beautiful struc¬ 
ture of the rosette that supports the ambu- 
lacral suckers of the Echinus. 
There can be no reasonable doubt that this 
peculiar arrangement is universal throughout 
the group, since it has been detected in cha¬ 
racteristic examples of every one of its prin¬ 
cipal subdivisions. And, consequently, as no 
similar calcareous reticulation is found in the 
internal or external skeleton of any other 
animal, even the minutest fragment which 
distinctly presents this structure may be re¬ 
ferred with certainty to an Echinoderm. 
And this structure is perfectly preserved, even 
after the substance has been infiltrated with 
calcareous matter in the act of fossilization, 
and has become so completely mineralised, 
that the disposition to rhomboidal fracture 
makes it difficult to obtain a section in any 
other direction than that of the plane of 
cleavage. As already remarked, the elemen¬ 
tary structure is essentially the same every¬ 
where ; so that it might not be possible to 
determine from a very minute fragment whe¬ 
ther it formed part of the shell of an Echinus, 
Cidaris, or Spatangus,—a portion of the frame¬ 
work of an Asterias, Ophiura, or Holothuria, 
— or entered into the composition of the stem 
of an Encrinite. But where any regular pat¬ 
tern is displayed, this is frequently sufficient 
to distinguish the genus, or even the species, 
to which the fragment belonged. This is 
certainly the case in regard to the spines of 
Cidarites and the stems of Pentacrinites ; and 
will probably be found no less true in other 
instances, when these beautiful structures 
shall have been more extensively investigated. 
Crustacea. — The structure of the shell in 
Crustacea has been hitherto examined only in 
the Decapod order ; and that of the common 
crab ( Platycarcinus pa gurus) alone has been 
subjected to a minute investigation. It is in 
the Decapod order that the shell attains its 
most perfect development, and contains the 
largest proportion of mineral matter : the 
special respiratory apparatus in this order 
being so elaborate as to render unnecessary 
any participation of the general tegumentary 
surface in the function of respiration. (See 
vol. i. p. 752.) 
The shell of the Decapod Crustacea con¬ 
sists of three layers;—namely, 1. a horny 
epidermic membrane covering the exterior ; 
2. a cellular or pigmentary stratum ; and 3. a 
calcareous or tubular substance. The horny 
epidermic membrane is easily detached from 
the subjacent layers, after the shell has been 
immersed for a time in dilute acid ; it is thin 
but tenacious, presenting no trace of structure, 
though it may exhibit markings on the under 
surface, derived from its contact with the cel¬ 
lular layer beneath. The pigmentary stratum 
is very thin in the crab and lobster ; but in 
some other Decapods it is much thicker. In 
Scyllurus latus, it is stated by M. Lavalle to be 
the thickest of the three layers of the shell ; 
and in the cray-fish and many other species, 
according to the same observer, it seems made 
up of a considerable number of layers, its ver¬ 
tical section being traversed by several ex-
        

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