Mr. C. Spence Bate's Notes on Crustacea. 297 



I may say, from its peculiarity, is decisive, independent of the 

 host of facts above mentioned, that Mr. Alder's genus is unte- 

 nable, one of its species being, as I stated in a late paper on the 

 PyramidellidcB, a decided Chemnitzia. As the discovery of the 

 process in Chemnitzia has now irrevocably satisfied me that the 

 genus Jeffreijsia is superfluous, I will not, as I have proposed in 

 the autumn, trouble you in reference to Mr. Alder's memoir. I 

 will in a subsequent paper oflfer a curious statement relative to 

 the Chemnitzice, resulting from the present investigation. 



XXVIII. — Notes on Crustacea. By C. Spence Bate. 



[Continued from vol. vi. p. 111.] 



[With a Plate.] 



On the Fifth pair of Legs in the Anomoura. 



IV. The fifth pair of legs, which both in the Brachyura and 

 Macroura are attached to the last thoracic ring, in the Ano- 

 moura belong to the first abdominal ring. Like all the others 

 they consist of not less than six joints, though sometimes the 

 last is so short, that with a process of the penultimate it com- 

 bines to form a didactyle claw having a prehensile power, similar 

 to the more efficient forceps of the first paii-. 



Although they consist of a similar number of joints to the 

 other pairs, and are in many instances nearly equal to them also 

 in length, yet they are powerless and not at all adapted to assist 

 in walking ; in fact, their common position is to be folded up 

 and at rest upon the back. But though inefficient for the ordi- 

 nary purposes of legs, they yet fulfill a successful part in the 

 oeconomy of the creature, and are useful for many purposes. 



In each of the other tribes of the decapod crustaceans, the 

 brauchise are supplied with organs especially adapted for the pur- 

 pose of keeping them free from the lodgement of foreign particles, 

 and also to excite currents over then- surfaces. These offices, which 

 in the Brachyura and Macroura are performed by the flabellse, are 

 in the Anomoura accomplished by the fifth pair of legs, which, 

 when necessary, are inserted into the branchial chamber ; to faci- 

 litate which, all this tribe have a peculiar articulation of the cara- 

 pace which gives them the power of raising their shell. This 

 power they avail themselves of in order to admit to the branchiae 

 as large a body of aerating fluid as possible, when circulation of 

 the blood has become impeded, as for instance when they have 

 been for any length of time confined in a small quantity of water : 

 a membrane connecting the carapace with the inner walls of the 



