10G 1SOPODA. 



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.-t.) are generally furnished 

 with true branchiae attached to the posterior surface of 

 the inner plate. The arrangement of these organs in 

 the Anceidae 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 lone 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 (Sphseroma, &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 



