106 ISOPODA. 
ceous plates, membranous in their texture, and arranged 
so that one lies upon and protects the other, the inner of 
which is generally more delicate than the outer one, 
and the two penultimate (s.-¢.) are generally furnished 
with true branchie attached to the posterior surface of 
the inner plate. The arrangement of these organs in 
the Anceide is very remarkable, the two pairs of plates 
attached to each of the five basal segments being small 
and free, as in Squilla, the tail, also, being terminated by 
a five-plated apparatus. This peculiarity in the branchial 
organs formed the subject of a communication made by 
Professor Westwood to the British Association, in the 
year 1832 (p. 593). 
In the female of the remarkable genus Ione each of 
the five basal segments of the tail is furnished with a pair 
of long branching very slender filaments, somewhat re- 
sembling a piece of coral in miniature; whether these be 
employed as respiratory organs has not been observed, 
the long filiform appendages attached to the three ante- 
rior pairs of legs having been regarded as employed in 
breathing. The posterior pair of pleopoda always, how- 
ever, differ from the preceding pairs, and either form a 
pair of strong valves, shutting together in a straight line 
so as to cover and protect the other respiratory plates, as 
in Idotea, or are transformed into a pair of double or 
single branched crustaceous plates (Spheroma, &c., Onis- 
cus, &c.), forming, with the central apical plate of the 
tail, a fan-like appendage of three or five pieces, similar 
to that which terminates the body of the macrourous deca- 
poda, or these lateral appendages are elongated into long 
terminal filaments which sometimes, as in Ligia italica 
and Apseudes talpa, equal the entire body in length. 
The majority of the Isopoda being aquatic, respira- 
tion takes place by means of the pleopoda transformed 
