278 CRUSTACEA. 



Ganunarus pulex is very common in fresh water. Other 



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



fleas " (Talitrus and Orchestia] are exceedingly abundant. 



On solid ground they 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. 

 Caprella, a common marine gymnast on Hydroids, &c., has 



the trunk of the body reduced to the quaintest possible 



minimum. 



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 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. 

 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 

 uniformly 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. 



Order 4. Decapoda. The shield is large and firm, and is fixed to the 



dorsal surface of all the thoracic segments. Of the thoracic 



appendages, the first three pairs are maxillipedes, the five other 



pairs are jointed walking legs (whence the term Decapod). 



Sub-order I. Macrura. Abdomen long. Homarus (lobster) ; 



Nephrops (Norway lobster, sea crayfish) ; Astactis (fresh 



water crayfish) ; Palinurus (rock lobster), whose larva was 



long known as the glass crab (Phyllosoma) ; Pencsus, a 



shrimp which passes through Nauplius, Zoeea, and Mysis 



stages ; Lucifer and Sergestes are also hatched at a stage 



antecedent to the Zooea ; Crangon vulgaris (the British 



