244 



MANUAL OF ZOOLOGY. 



composed 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 number of seg- 

 ments in the abdomen varies, but is never more than seven. 

 The abdominal segments are coalescent, and form a broad 

 caudal shield, beneath which the branchiae are carried. The 

 eyes are two in number, formed of a collection of simple eyes. 

 or sometimes truly compound. The heart is sometimes an 

 elongated tube, with three pairs of fissures (as in the Amphi- 

 poda], sometimes short or spherical, removed towards the 

 abdomen, and with more or fewer fissures than the above. 

 The young Isopod is developed within a larval membrane, 

 destitute of appendages. After a time this membrane bursts, 

 and liberates the young, which resembles the adult in most 

 respects, but possesses only six instead of seven pairs of limbs. 

 Of the members 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 respectively, from their habits, the Natatorial, 

 Sedentary, and Cursorial Isopods. In the Natatorial Isopoda 

 the extremity 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 

 \Cymothod), whilst others 

 are found in the sea {Sphce- 

 roma). In the Sedentary Iso- 

 poda the animals are all pa- 

 rasitic, with short, incurved, 

 hooked feet. This section 

 includes the single family 

 of the Bopyrida, all the 

 species of which live para- 

 sitically 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 run- 

 ning, Isopods mostly live 



Fig. 94. Isopoda. Wood-lice (Oniscus). 



upon the land, and are therefore destitute of swimming feet. 

 The most familiar examples of this section are the common 



