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/lit25759/790/
778 
CRUSTACEA. 
this, striæ are observed to appear, which are 
the rudiments of the branchial filaments. 
During this interval the thoracic extremities 
have become developed, and above their bases 
other branchiae have made their appearance, 
presenting in the beginning the form of tuber¬ 
cles, and subsequently that of stilets ; smooth 
and rounded on their surface, but by-and-by 
becoming covered with a multitude of small 
tuberculations, which by their elongation are 
gradually converted into branchial filaments 
similar to the preceding. During this period 
of the development of the branchiae these 
organs are applied like the extremities to the 
inferior surface of the embryo ; but they sub¬ 
sequently rise against the lateral parts of the 
thorax, become lodged within a cavity situated 
under the carapace, and thus are no longer 
visible externally. 
The cavity destined to protect in this manner 
the branchial apparatus, is neither more nor 
less than an internal fold of the common tegu¬ 
mentary membrane. It shows itself first under 
the guise of a narrow groove or furrow, which 
runs along the lateral parts of the thorax below 
the edge of the lateral piece of the carapace. 
This longitudinal furrow is not long of expand¬ 
ing, and becomes consolidated by its superior 
edge with the internal surface of the carapace, 
which, by being prolonged inferiorly, consti¬ 
tutes the external wall of a cavity, the opening 
of which, situated above the base of the 
extremities, becomes more and more contracted, 
and ends by being almost entirely closed. The 
space in this way circumscribed encloses the 
branchiae, and constitutes what is called the 
respiratory cavity of the Decapod Crustaceans. 
From what has just been said, it would ap¬ 
pear that the embryo of the Astacus fluviatilis 
presents four principal periods with reference 
to the state of the respiratory apparatus ; lstly, 
that which precedes the appearance of this ap¬ 
paratus ; 2dly, that during which the branchiae 
are not distinguishable from the flabelliform ap¬ 
pendages of the extremities, or in which it 
consists of simple lamellar or stiliform pro¬ 
cesses, which appear as mere processes of 
other organs especially dedicated to locomotion 
or to mastication ; 3dly, that characterized by 
the transformation of these extremely simple 
appendages into organs of a complex structure, 
entirely distinct from the extremities, but still 
entirely external ; 4thly and lastly, that during 
which the branchiae sink inwards and become 
lodged in a cavity especially adapted for their 
reception, and provided with a particular 
apparatus destined to renew the water neces¬ 
sary to the maintenance of respiration. 
If we now turn to the examination of the 
apparatus of respiration in the different groups 
in which it exhibits important modifications, 
we shall, in the series of Crustaceans, encounter 
permanent states , analogous to the various 
phases through which we have just seen the 
apparatus passing in the most elevated animals 
of the class. 
And, in fact, the first period which we have 
particularized above in the embryonic life of the 
Decapod is exhibited in the permanent condi¬ 
tion of some inferior Crustaceans, in which not 
only is there no special organs for respiration, 
but in which none of the appendices occur 
with such modifications of structure as would 
fit them to become substitutes for the branchiae, 
in which, consequently, the process of respira¬ 
tion, that is the aeration of the blood, appears 
to take place over the surface of the body at 
large. The greater number of the Haustellate 
Crustacea, of the Entomostraca properly so 
called, of the Copepoda, and even of the 
Phyllosomata, appear to belong to this type 
of organization. 
A state analogous to that which characterizes 
the second period in the development of the 
embryo of the Decapod, is presented to us in 
a large number of other Crustaceans, the orga¬ 
nization of which is more perfect than that of 
the animals of which mention has just been 
made, we mean the Branchiopoda and Edri- 
ophthalmia, in which, although we do not yet 
find branchiæ properly so called, that is to 
say, organs peculiarly devoted to respiration, 
we discover certain appendages of the extre¬ 
mities which serve for this function. In the 
Branchiopoda {fig. 421) the whole of the tho¬ 
racic extremities present 
a lamellar conformation, Fig. 421. 
and the two external 
portions of the appen¬ 
dages corresponding to 
the palp and flabellum 
(fouet), form membra¬ 
nous vesicles of a flat¬ 
tened form, soft to the 
touch, and highly vas¬ 
cular, the structure of 
which appears eminently calculated to facilitate 
the action of the air upon the nutritious fluid. 
(6, c,fig. 421). 
In the Amphipoda another step appears to 
be taken in the elaboration of the respiratory 
apparatus. Not only does the function of 
respiration tend to become centred in certain 
appendages, whose structure is modified for 
this end,“ but this localization, if the term may 
be allowed, becomes more complete ; for the 
two appendicular portions of the thoracic 
extremities no longer concur indistinctly and 
Fig. 422. 
vicariously in the performance of the function ; 
the palp {b, fig. 422) has other uses apportioned 
to it, and the flabellum (c) alone plays the part 
of the branchiæ. These appendages, in other re¬ 
spects, do not present any thing peculiar in their
        

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