ANNULOSA: CRUSTACEA. 



213 



and consist of branchiae, which in the terrestrial species are 

 protected by plates which fold over them. The thorax is com- 

 posed of seven segments, 

 bearing seven pairs of limbs, 

 which, in the females, have 

 marginal plates, attached to 

 their bases, and serving to 

 protect the ova. The num- 

 ber of segments in the ab- 

 domen varies, but is never 

 more than seven. The eyes 

 are two in number, formed 

 of a collection of simple 

 eyes, or sometimes truly 

 compound. Of the mem- 

 bers of this order, many are 

 aquatic in their habits, and 

 are often parasitic, but others 

 are terrestrial. 



By Milne - Edwards the 

 Isopoda are divided into 

 three sections, termed re- 

 spectively from their habits, 

 the Natatorial, Sedentary, 



Fig. 70. Isopoda. Wood-lice (Oniscus. ) 



and Cursorial Isopods. In the Natatorial Isopoda the ex- 

 tremity of the abdomen and the last pair of abdominal legs are 

 expanded so as to form a swimming tail. Some of this section 

 are parasitic upon various fishes ( Cymothoa), whilst others are 

 found in the sea (Sphceroma). In the Sedentary Isopoda the 

 animals are all parasitic, with short, incurved, hooked feet. 

 This section includes the single family of the Bopyridce, all the 

 species of which live parasitically either in the gill-chambers, 

 or attached to the ventral surface, of certain of the Decapod 

 Crustacea, such as the Shrimps ( Crangones) and the Palcemones. 



The Cursorial, or running, Isopods mostly live upon the land, 

 and are, therefore, destitute of swimming feet. The most 

 familiar examples of this section are the common Wood-lice 

 (Oniscus]. Here, also, belongs the little Limnoria terebrans, 

 so well known for the destruction which it produces by boring 

 into the wood-work of piers and other structures placed in 

 the sea. 



DIVISION B. PODOPHTHALMATA. The members of this divi- 

 sion have compound eyes supported upon movable stalks or 

 peduncles, and the body is always protected by a cephalotho- 

 racic carapace. It comprises the two orders Stomapoda and 



