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 branchiw, 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 branchim 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 branchiw 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 nol,v 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 branchia,, 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 branchim 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, _ _ and the two external portions of the appen dages corresponding to the palp and flabellum Cfouet), form membra nous vesicles of a flat tened form, soft to the touch, and highly vas , cular, the structure ot which appears eminently calculated to facilitate the action of the air upon the nutritious fluid. (b, c,fig. 4.21).
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 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 branchi. These appendages, in other re spects, do not present any thing peculiar in their conformation ; they appear like a vesicular or foliaceous expansion, of an extremely soft tex ture, which is attached to the inner edge of the base of the thoracic extremities; their dimensions generally increase from before back wards, and the last pair of thoracic extremities is not furnished with any : their total number varies from eight to twelve. These organs, suspended under the thorax, float in the ambient fluid, and the water in contact with their surface is incessantly renovated by means of the motions performed by the abdominal extremities of the animal, motions which occa sion a rapid current from behind forwards along the ventral aspect of the body.