Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25759/768/
CRUSTACEA. 
fectlv distinct, and differ but little from one 
another; and the seven abdominal rings, in 
like manner, remain moveable, and only differ 
from those of the thorax as they do from one 
another by a relatively inferior degree of deve¬ 
lopment. In the majority of the Isopods the 
structure of the tegumentary skeleton is essen¬ 
tially the same as in the Amphipods ; but there 
occurs a greater inequality of development be¬ 
tween the thoracic and the abdominal rings, 
most of the latter remaining more or less in a 
rudimentary state. 
In the Apus and the Nebalia we conti¬ 
nue to find the rings of the thoracic and abdo¬ 
minal portions of the tegumentary skeleton 
nearly equal in size and similar in form ; but 
the cephalic section, instead of presenting the 
same conformation as these two portions of the 
body, constitutes superiorly an immense shield, 
which extends over the rings of the thorax and 
conceals them. This dorsal shield or buckler, 
which is denominated Carapace by zoologists, 
also occurs among the whole of the Podoph- 
thalmians, and more than all besides conspires 
to give to these animals their distinguishing 
peculiarities of shape. Inquiries, of which it 
would be tedious to give a detailed account in 
this place, have led us to discover that the 
carapace of these Crustaceans is neither more 
nor less than the superior arc of the third or 
fourth cephalic ring, enormously developed, 
and which in attaining its large dimensions 
laps over and modifies the conformation of a 
greater or smaller number of the neighbouring 
rings.* 
In the generality of the Stomapods the cara¬ 
pace does not quite cover and conceal the two 
first cephalic rings, which indeed continue dis¬ 
tinct and moveable ; but in the whole of the 
Decapods these rings cohere with one ano¬ 
ther and with the following ones, and unite 
more and more intimately under the carapace, 
which then covers the whole of the head as well 
as the thorax. In the Macroura the anterior 
extremity of the carapace only extends over the 
ophthalmic or first cephalic ring; but in the 
Brachyura it bends around this ring so as to 
include it, and to go to unite underneath with 
the next segment. As we ascend in the series 
of Crustaceans, we observe the carapace en¬ 
croaching more and more upon the thorax. 
In the Squillæ the three last cephalic and three 
first thoracic rings are nearly lost by becoming 
blended with those to which the carapace be¬ 
longs ; they scarcely retain any mobility, and 
protected above by this shield, unite intimately, 
and remain imperfect in their tergal portions ; 
the four last rings of the thorax continue, on the 
contrary, free, and are in almost every particular 
similar to those of the abdomen. In the Mysis 
this union of the cephalic shield with the seg¬ 
ments of the thoracic division of the tegumen¬ 
tary skeleton is carried further, for there are 
not more than two of these rings which remain 
distinct. But it is in the Decapods that the 
carapace attains its greatest development, and 
* See my Hist. Nat. des Crustacés, t. i. p. 23. 
that its influence upon the evolution of the 
thoracic segments is carried the farthest. 
In these animals the framework of the body 
does not appear at first sight to consist of more 
than two portions, the one anterior, formed by 
the carapace, and representing the cephalic and 
thoracic segments conjoined ; the other poste¬ 
rior, formed by the abdomen. In reality, the 
first fourteen rings of the body are covered by 
this enormous buckler, and are so intimately 
conjoined as to have lost all their mobility; the 
whole of the thoracic segments thus hidden 
below the carapace, are connected with it in 
their superior part, they are only joined with 
one another underneath and laterally ; and their 
tergal parts having, in consequence of this, be¬ 
come useless, are no longer to be found, being 
in some sort replaced by the great cephalic 
buckler; thus the whole of these rings, in con¬ 
formity with this arrangement, are imperfect 
and open above. 
Hitherto we have not been able to deter¬ 
mine whether the carapace of the Podophthal- 
mia is formed at the expense of the third or 
of the fourth ring of the tegumentary skeleton ; 
but we have the strongest reasons to conclude 
that this buckler is neither more nor less than 
the dorsal arc of one or other of these cephalic 
rings, and not of the two conjointly. In fact 
we cm here demonstrate a composition analo¬ 
gous to that which we have already pointed 
out as characteristic of every arc, whether supe¬ 
rior or inferior, of the different rings in their 
state of complete development, to wit, a tergal 
portion and two lateral or epimeral pieces. In 
following the embryo of the River-crab in its 
progressive stages of development, Rathke* 
observed the carapace to be formed of three 
pieces, which at length became consolidated 
so as to form but one. In many of the Deca¬ 
pods it is even easy to perceive this structure 
or composition in the carapace of adults, inas¬ 
much as there exist lines marking the conjunc¬ 
tion, and accurately indicating the respective 
limits of the different pieces of which this great 
dorsal plate is composed. 
The general form of the carapace depends in 
great measure on the relative development of 
these different pieces; in the Macroura the 
tergal portion of the carapace extends but a 
short way backwards, whilst the lateral or 
epimeral pieces reach as far as the begin¬ 
ning of the abdomen, and being no longer 
kept at a distance by the tergum, meet in the 
median line of the back, and are there con¬ 
joined. In the Brachyura, on the contrary, the 
tergal portion is that which is especially deve¬ 
loped, so that it constitutes the whole of the 
upper part of the carapace, whilst the lateral 
pieces, thrust outwards and underneath, only 
form a narrow band above the bases of the ex¬ 
tremities. 
It is also in consequence of modifications 
analogous to those on which the existence of the 
carapace depends, that in other Crustacea the 
* Untersuchungen ueber die Bildung des Fluss¬ 
krebses, &c. Tr. in Annales des Sciences Nat. t. 20.
        

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