CRUSTACEA. 



779 



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. 



In the Lremodipoda, the parts which perform 

 the office of branchiae are vesicular bodies 

 formed by the flabelliform appendage of a 

 certain number of the pairs of thoracic extre- 

 mities. In the Isopoda, finally, the locomotory 

 extremities no longer serve for respiration, the 

 function being committed to the five first pairs 

 of abdominal extremities which are entirely 

 devoted to it and cease to have any other uses. 

 These extremities, which are designated under 

 the name of false branchial limbs, consist of a 

 cylindrical articulation, supporting two folia- 

 ceous, soft membranous laminae, vascular in a 

 greater or less degree ; frequently, too, we 

 perceive on their inner side a small appendage, 

 which may be regarded as analogous to the 

 femur or stem of the other extremities, whilst 

 the two laminae, of which mention has just 

 been made, appear to represent the palp and 

 the flabellum. In the greater number of Iso- 

 poda these organs are completely exterior, but 

 in several (such as the Idotea) the last ring 



Fig. 423. Fig. 424. 



Respiratory apparatus of the Idotea. 



of the abdomen supplies them with a cavity, 

 the entrance to which is closed by valves 

 which constitute the two appendages of the 

 same ring. 



The Stomapoda which have already supplied 

 us with an instance of the absence of deter- 

 minate organs of respiration, also exhibit 

 something analogous to the transition state of 

 this apparatus during the second period of the 

 embryonic life of the Decapod. In the genus 

 Cynthia the branchiae are represented by a 

 small membranous cylinder, attached by its 

 middle to a peduncle, itself implanted upon 

 the extremity of the basilar articulation of the 

 five first pairs of abdominal extremities. 



The third type of the respiratory apparatus 

 specified above, is presented to us by other 



Stomapods, known under the names of Squillae 

 and Thysanopodse. In these creatures, in fkct, 

 we discover branchioe properly so called, the 

 structure of which is greatly complicated, 

 more so even than in the Crustaceans at the 

 very head of the series ; still the respiratory 

 apparatus as a whole is much less complete, 

 for they are not included in a cavity, and float 

 freely in the water which bathes the entire sur- 

 face of the body of the animal. In the Squillae 

 (JigA'25) the branchiae are attached to the basilar 

 joint of the first five pairs of abdominal extremi- 



Fig. 425. 



One of the branchier 

 of the Sijiiitla. a, 

 branchia fixed to the 

 abdominal extremity 

 (b). 



ties, and each consists of a long cylindrical 

 tube, upon one of the sides of which proceeds 

 a series of small tubes disposed parallel to one 

 another like the pipes of an organ and support- 

 ing in their turn a series of long cylindrical and 

 very numerous tubes. * In the Thysanopoda 

 the branchiae also resemble plumes, but in- 

 stead of being situated on the abdomen, they 

 are attached to the thoracic extremities. f 



Finally, the last or highest term of develop- 

 ment which we have mentioned in the River- 

 crab, is also presented to us by the entire order 

 of Decapod Crustaceans. Not only is the func- 

 tion of respiration thrown upon particular 

 organs, created expressly for this purpose, in 

 the whole of these animals, but further, the 

 organs themselves are lodged and protected 

 within especial cavities, and the renewal of the 

 water necessary to their functions is secured by 

 the action of distinct appendages belonging 

 more particularly to the masticatory and loco- 

 motory apparatuses. 



Let us now take a survey of the branchial 

 cavity. It occupies (fig. 426J the lateral part 

 of the thorax, and extends between the vault of 

 the flancs and the lateral portion of the cara- 

 pace, from the base of the extremities all the 

 way towards the dorsal aspect of the animal. 

 As we have already said, it is formed by an in- 

 ternal fold of the common tegumentary mem- 

 brane, which, after having formed the vault of 

 the flancs, re-descends towards the base of the 

 extremities to become continuous with the 

 carapace. The internal and inferior wall of 

 this cavity is consequently formed by the vault 

 of the flancs itself, and its external and superior 

 wall by a membranous septum, which in the 

 greater part of its extent is for the most part 

 connected with the corresponding portion of 



* Cuvier, Lemons d'Anat. comp. t.iv. 



t Mem. sur une disposition particuliere de 1'ap- 

 pareil branchial chez quelquesCrustaces, par Milne 

 Edwards, Ann. des Sciences Nat. torn. xix. 



3 E 2 



