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 branchiz 
(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 Ligiidz the six segments are all distinct, in the 
Asellidze 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 
Ione, 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 Cymothoide (Régne An., Ed. Crochard, Crust., pl. 59, fig.1). It appears 
to us to be most nearly allied to the Bopyride, especially to the recently 
established genera Athelgue and Prosthete. 
