INTRODUCTION. XXvii 



last or flabelliform appendage of the second pair of the 

 true jaws, which is developed into a broad horny plate, 

 fixed by a sort of pivot, on which it continually turns, 

 and thus regulates the efflux of the water. Prof. Milne 

 Edwards observes, that this action is proved to be es- 

 sential to the renewal of the water which bathes the 

 branchiae, as, if its movements be stopped, the animal 

 becomes soon asphyxiated. The whole of the apparatus 

 belonging to this function in the higher Crustacea is 

 exceedingly curious and interesting, but it would be out 

 of place to enter into the detail in this work. 



The branchiae are very differently formed in the dif- 

 ferent orders of the class, and even vary considerably in 

 some genera of the same family. In some cases the 

 abdominal appendages support these organs; in others 

 they are attached to the basal joint of the thoracic legs; 

 in some genera, as in My sis, their distinct existence has 

 not as yet been demonstrated, although, as I have ob- 

 served in speaking of that genus in the body of the work, 

 there appears little doubt that a special organ exercises 

 their function. 



In the terrestrial Isopoda, or the common Millipedes, 

 as they are termed, the respiration is exclusively atmo- 

 spheric. 



The respiration of the land crabs, which must neces- 

 sarily be, during the greater part of their lives, atmo- 

 spheric, is one of the most remarkable phenomena 

 connected with this subject, and has occupied the atten- 

 tion of Mons. Audouin and Dr. Milne Edwards, who have 

 given a most elaborate and interesting memoir on this 

 subject,* to which the reader is referred. It is well 



* Annales des Sciences Naturelles, t. v. p. 85. 



