778 



CRUSTACEA. 



this, striae 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; Istly, 

 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 branchiae 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 

 (foitet), 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. 

 (b, c,Jig. 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. 



421. 



vicariously in the performance of the function ; 

 the palp (&,.,%. 422) has other uses apportioned 

 to it, and tlie flabellum (t) alone plays the part 

 of the branchiae. These appendages, in other re- 

 spects, do not present any thing peculiar in their 



