MALACOSTRACA. 271 



to the head, the corresponding appendages serve as maxillipedes, the 

 other thoracic segments (seven or six) are free. The eyes are sessile. 

 The heart is elongated. 



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, Apseudes, etc., 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 over- 

 lapped 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. Arma- 

 dillo), which roll themselves up, are called "pill-bugs." 

 In the terrestrial forms there is obviously a departuie 

 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 shoie rocks. 

 The "gribble" (Limnoria lignorum) is a destructive 

 marine Isopod which eats into wood. 

 Among the marine Cymothoidae 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 Bopyridee infest the gill-chambers of other Crustaceans, 

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

 by their mates. 

 Among the parasitic Cryptoniscidte we again find herma- 

 phrodites with associated pigmy males. In not a few 

 cases they seriously affect the reproductive organs of their 

 male hosts. 

 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" 

 (Cyamidce), two segments are involved. The thoracic limbs 

 bear respiratory appendages. Of the six pairs of legs which the 

 abdomen usually bears, the anterior three are usually more 

 strongly developed as swimmers, while the posterior three — 

 directed backwards — are used in jumping. 



Cammarus pulex is very common in fresh water. Other 

 species occur on the seashore. There also the "Beach- 

 fleas " ( Talitrus and Orcheslia) are exceedingly abundant. 



