ISOPODA. 105 



developed as the corresponding pair of legs of Squilla, 

 which bears a large pair of sabre-like claws ; indeed, 

 this arrangement of the limbs in Arcturus very satisfac- 

 torily settles the question of the homologues of the limbs 

 of Squilla, which has been the subject of some discussion. 

 In the true Isopoda, however, the three anterior pairs of 

 legs are directed forwards, and the four posterior pairs 

 backwards, thus differing from the Amphipoda. 



In the female Isopoda the legs are provided at the base 

 with a series of large membranous plates, arranged so as 

 to lie in a horizontal position, overlapping one another, 

 and together forming a pereionic pouch within which the 

 eggs are lodged during incubation, and the young are re- 

 tained for some time after they are hatched. The respira- 

 tory apparatus of these animals being different from that of 

 the Amphipoda, the vesicular appendages, which exist at 

 the base of the legs of the latter, serving as branchiae 

 (vol. i. fig. * 1'"), are not found in the Isopods,* their duty 

 as organs of breathing being transferred to, and performed 

 by, the organs attached to the pleopoda (tail-legs), modi- 

 fied in various ways. The tail (pleon) is always short, 

 but well developed ; the six joints of which it is com- 

 posed are often more or less fused together ; thus, whilst 

 in the Ligiidce the six segments are all distinct, in the 

 Asellidse they are almost reduced to one very large 

 terminal plate. The five anterior pairs of appendages 

 attached to the underside of these tail segments are em- 

 ployed as respiratory organs, and consist of a peduncle, 

 bearing at its extremity two large oval, movable, folia- 



* Except, as before remarked in the genus Tanais, and also in the genus 

 lone, which Latreille was thence induced to place in the order Amphipoda, 

 but which Prof. Milne Edwards considered to belong to the Isopoda, allied to 

 the Cymothoidaj (Regno An., Ed. Crochard, Crust., pi. 59, fig.l). It ajjpears 

 to us to be most nearly allied to the Bopyridse, especially to the recently 

 established genera Athelgue and Prosthete. 



