90 BRANCH ARTHROPODA 



CLASS I. CRUSTA'CEA 



As examples of this class may be named crayfishes, lobsters, 

 crabs, and " pill-bugs." The body has a limited number of 

 segments, about twenty in the crayfish. Each pair of append- 

 ages is regarded as being attached to a different segment. 

 The head and thorax are united and called cephalothorax. 

 The chitinous covering, rendered hard by deposits of carbonate 

 and phosphate of lime, is called the carapace. 



Respiration is by gills, or branchiae, though some breathe 

 through the skin. 



The appendages are biramous, as seen in the swimmerets of 

 the crayfish. A typically developed appendage, as the third 

 pair of swimmerets, consists of a main stalk (protopod) and two 

 branches, the outer (exopod) and the inner (endopod). Several 

 of the appendages lack some of these parts. The student should 

 homologize the appendages and tell or demonstrate which ones 

 have missing parts. 



The class Crusta'cea is usually divided into two sub-classes, 

 the En'tomos'traca and the Mal'acos'traca, with several orders 

 under each. 



Sub-class Entomostraca is composed of crustaceans with a 

 varying number of joints or segments. They are usually small 

 or microscopic. There is a metamorphosis, the first stage being 

 the free-swimming nauplius. 



Order I. Phylldp'oda are small aquatic crustaceans with 

 segmented bodies and leaf-like appendages. The brine shrimp, 

 fresh-water Branchipus, and Daphnia are examples of the 

 order. Daphnia is shelled and looks like a very small clam. 



The animals of this order form an important part of the 

 food of fresh-water fishes. The eggs of many species can 

 resist the drought, which is a valuable means of perpetuating 

 them in small streams which dry up in summer. 



Order II. Ostrac'oda are small crustaceans with apparently 

 unsegmented bodies enclosed in a bivalve shell, as the fresh- 

 water Cypris. The abdomen is rudimentary. There are only 

 two pairs of thoracic appendages, two pairs of maxillae, one pair 

 of mandibles, one pair of antennae, and one pair of antennules. 

 The antennae and antennules are used for locomotion. The 



