CLASSIFICATION OF CRUSTACEA. 277 



Order I. Anisopoda. The fusion of the first two thoracic segments 

 to the head, the presence of a cephalothoracic shield, and other 

 divergent features distinguish Tanais, Apsetides, 6<:., from the 

 Isopoda. 



Order 2. Isopoda. The body is flattened from above downwards. 

 The first thoracic segment is fused to the head, while the other 

 six or seven are free, and there is no cephalothoracic shield. The 

 abdomen is usually short, and its appendages, usually overlapped 

 by the first pair, are plate-like, and function in part as respiratory 

 organs. 



^=*The "wood lice" (Oniscus, Porcellio) are familiar animals 

 which lurk in damp places under stones and bark, and 

 devour vegetable refuse. Some related forms (e.g., 

 Armadillo), which roll themselves up, are called " pill 

 bugs." In the terrestrial forms there is obviously a 

 departure from the ordinarily aquatic habit of Crustaceans, 

 and the exopodites of some of the abdominal appendages 

 have tubular air passages. 

 Asellus is a very common form, living in both fresh and salt 



water. 



Idotea is not uncommon among the shore rocks. 

 The "gribble" ( Linmoria lignorum) is a destructive marine 



Isopod which eats into wood. 



Among the marine Cymothoidse which are often parasitic on 

 fishes, some, e.g., Cymothoe, are remarkable in their sexual 

 condition, for they are hermaphrodites in which the male 

 organs mature and become functional when the oviducts 

 are still closed, while at a later period in life the male 

 organs are lost and the animals become functionally 

 female. 



The Bopyridre infest the gill chambers of other Crustaceans, 

 e.g., prawns. The pigmy males are usually carried about 

 by their mates. 



Among the parasitic Cryptoniscidce, we again find herma- 

 phrodites with associated pigmy males. In not a few 

 cases they seriously affect the reproductive organs of their 

 male hosts. 



Many of these Isopods, like not a few other Crustaceans, are 

 extremely interesting to those who care to think about the 

 problem of sex. Thus, to cite one other instance, the males and 

 females in the genus Gnathia are so unlike, that they have been 

 mistakenly referred to different sub-families. 



Order 3. Amphipoda. The body is laterally compressed. In most 

 it is only the first thoracic segment which is fused to the head, in 

 the " no-body-crabs " (Caprellida), and "whale lice" (Cyamida}, 

 two segments are involved. The thoracic limbs bear respiratory 

 appendages. Of the six pairs of legs w 7 hich the abdomen usually 

 bears, the anterior three are usually more strongly developed as 

 swimmers, while the posterior three directed backwards are 

 used in jumping. 



