ARTHROPODA. 85 



Class I. Crustacea. 



Aquatic forms usually breathing by gills, with two pairs of antennae, 

 with paired limbs on the thorax and usually on the abdomen. 



On the head two pairs of antennae, a labrum, two mandibles with 

 palps, and two pairs of maxillae. The thorax bears (at least three) pairs 

 of appendages, of which some of the most anterior are frequently modified 

 into foot-jaws (maxillipedes), while the following vary in form and 

 function, as do the abdominal appendages. The typical larval form of 

 the lower Crustaceans is the Nauplius, of the higher the Zoea. 



GROUP A. ENTOMOSTRACA. Small forms of simple structure, with a 

 variable number of segments and appendages. Generally a Nauplius 

 larva. 



Order 1. Phyllopoda. With four or more pairs of leaf -shaped lobed 

 swimming feet, and generally a carapace or bivalve shell. Mostly in 

 freshwater lakes. 



Branchiopoda, relatively large, with saccular gills attached to 

 the numerous swimming feet. Elongated heart. Generally a nauplius 

 larva. Branchipus stagnalis. Apus cancriformis, male very rare. 



Cladocera, water fleas, laterally compressed and imperfectly seg- 

 mented, with large locomotor second antennae, generally with a bivalve 

 shell. Heart short. Summer and winter eggs ; parthenogenesis. Rarely 

 a larva. Daphnia pulex. Leptodora, nauplius from winter eggs. 



Order 2. Ostracoda. With unsegmented body, rudimentary abdomen, 

 and seven pairs of appendages. Enclosed in a bivalve shell. Cypridina 

 serrato-striata, fossil, C. mediterranea, recent. Cypris fusca. 



Order 3. Copepoda. Usually well segmented ; devoid of a shell. 

 Four to five pairs of biramous swimming feet on the thorax, abdomen 

 without appendages. Cyclops coronatus, without heart. Common in 

 fresh water. Ergasilus, Chondracanthus, Caligus, Lernsea, Lernse- 

 ocora, Achtheres, and Anchorella, degenerate parasites with piercing 

 or sucking mouth-parts and stunted abdomen. Argulus foliaceus on 

 the carp and stickleback ; the second maxillae are modified into suckers. 



Order 4. Cirripedia. Sessile hermaphrodites with a duplicature of the 

 skin (and a shell). Lepas anatifera, ship barnacle. Balanus tin- 

 tinnabulum, acorn barnacle, fossil and recent. 



As Rhizocephala or Suctoria we group together the parasitic 

 forms in which segmentation and gut are absent. Peltogaster paguri, 

 Sacculina carcini. 



