INTRODUCTION. 



XXXlll 



other general of the aberrant type, where it is situated, 

 according to the observations of Fritz Miiller, in the dorsal 

 surface of the pereion, corresponding in position with that 

 of the respiratory systems of the various orders. 



In the Amphipoda, the branchiae are by no means the 

 simple sacs that they have been described. They are 

 situated upon the inner surface of the coxse, and assume 

 the form of leaf-like hollow plates, ranged in parallel lines 

 on each side of the sternum (Fig. 10), and are attached to 

 every pair of legs except the first in 

 the females, and generally the last 

 in males ; though, in Gammarus, we 

 have seen the seventh pair fur- 

 nished with branchiaB as well as the 

 preceding. In the Aberrantia, the 

 number of sacs is reduced to two 

 or three pairs. In this order they 

 homologize with the branchiae of 

 the decapod type, each branchial 

 appendage being viewed in the light 

 of a single plate of the compound 

 organs of the higher type; or rather, 

 perhaps, they bear best comparison 

 with the same organ as it appears 

 in the larval condition in the 

 Brachyura. The great distinction 

 in their character is derived mostly 

 from the appearance which these 

 organs assume in the higher forms, 



FIG. 10. 



being that of an internal position. But this is one 

 of appearance only. The branchire are overcapped by 

 the monstrous production of the cephalic shield in the 

 Stalk-eyed orders of Crustacea, a circumstance that gives 

 to the portion of the dermal skeleton that it covers the 



c 



