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-
