272 



CRUSTACEA. 



On solid ground the}- move on their sides in a strange 



fashion, but they swim very swiftly. 

 Hyperia, Phronima, and many marine Amphipods, have a 



habit of living as commensals with other animals. 

 Cafrella, a common marine gymnast on Hydroids, etc., has 



the trunk of the body reduced to the quaintest possible 



minimum. (Fig. 116.) 



Legion 3. Thoracostraca. (Podophthahnata, 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 1. Cumacea. — The cephalothoracic 

 shield is small, and four or 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 flattened, 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 forming 

 swimming - paddles. The elongated 

 heart extends into the abdomen, 

 which also contains the reproductive 

 organs. The genera are marine, e.g. 

 Squilla. 



Fig. 116. — An Amphipod 

 (Caprella linearis). 



The two anterior thoracic 

 segments are fused to the 

 head ; the abdomen is 

 greatly reduced and with- 

 out appendages ; the fourth 

 and fifth thoracic segments 

 bear only respiratory 

 plates. 



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 posterior 

 thoracic segments. The eight thoracic appendages are uni- 

 formly biramose, but the first two may serve as maxillipedes. 

 The abdominal appendages of the male are strongly developed ; 

 those of the female are weak, except the last, which in both sexes 

 form paddles. They are marine forms, e.g. Mysis (without gills 

 on the thoracic legs), Lophogaster, and Euphausia (with gills on 

 the thoracic legs). The last-named starts in life as a Nauplius. 

 As an adult it has luminous organs on the eye-stalks, thoracic 

 legs, and abdominal segments. 



