CLASSIFICATION OF CRUSTACEA. 249 



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 Bopyridoe infest the gill-chambers of other Crustaceans. 

 The pigmy males are usually carried about by their mates. 

 Among the parasitic Cryptoniscidse, we again find herma- 

 phrodites with associated pigmy males. 

 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 

 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" (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 are used in jumping. 

 Gammanis pulex is very common in fresh water. 

 Other species occur on tlie sea-shore. There also the " Beach- 

 fieas" [Talitrus and Orchestia) are exceedingly abundant. 

 On solid ground they move on their sides in a strange 

 fashion, but they swim very swiftly. 

 Hyperia, Phroninia, and many marine Amphipods, have a habit 



of living as commensals with other animals. 

 Caprella, a common marine gymnast on sea-weeds, has the trunk 



of the body reduced to the quaintest possible minimum. 

 Cyamus is parasitic on the skin of whales. 



Legion 3. Thoracostraca. (Podophthalmata, with stalked eyes). 

 Several or all of the thoracic segments are fused to the head, and 



there is a cephalothoracic shield overlapping the gills. The two eyes 



are stalked except in Cumacea. 



Order i. Cumacea. The cephalothoracic shield is small, and four to 

 five thoracic segments are left uncovered and free. The eyes are 

 sessile and adjacent or fused. There are two pairs of maxillipedes. 

 The females have no abdominal appendages except on the last 

 segment. The genera are marine, e.g. , Cuma or Diastylis. 

 Order 2. Stomatopoda. The shield is still small and does not cover 

 the three posterior thoracic segments. The body is somewhat flat- 

 tened, the abdomen is very strong. Five anterior thoracic 

 appendages are directed towards the mouth and serve to catch 

 food, and to clamber. The five anterior abdominal legs carry 

 feathery gills, the sixth pair form swimming paddles. The 

 elongated heart extends into the abdomen, which also contains the 

 reproductive organs. The genera are marine, e.f;. , Squilla. 

 Order 3. Schizopoda. A delicate shield covers the whole of the 

 thorax, but there is still some freedom as to one or more of the post- 

 erior thoracic segments. The eight thoracic appendages are very 

 uniform, but the first two may serve as maxillipedes. The 



