104 ARTHROPODS 



While they are all alike in the characteristics necessary to 

 put them in a single group, a branch of the animal kingdom, 

 yet they differ sufficiently to put them in different classes. 



The branch to which these four types belong is Arthro- 

 poda, a word which refers to the segmentation of the legs. 



The class to which the crayfish belongs is Crustacea, a 

 word which refers to the covering of the animal. 



The crayfish belongs to the order Decapods, family As- 

 tacidae, and the Genus Cambarus. 



The following synopsis of the more common forms of 

 Crustacea is abridged from McMurrich's Invertebrate Mor- 

 phology. Examples are given under each division. 



CLASS CRUSTACEA 



1. Subclass Entomostraca. — Number of segments varies; abdo- 

 men without appendages ; larva a Nauplius. 



I. Order Phyllopoda. — Number of segments variable; appen- 

 dages with branchiae. 



1. Suborder Branciiiopoda. — Body plainly segmented and seg- 

 ments of thorax more numerous than six. 



Branchipus (sand flea). 



2. Suborder Cladocera. — Body indistinctly segmented ; with 

 bivalved shell; four to six thoracic appendages Daphnia. 



2. Order Ostracoda. — With bivalved shell ; body indistinctly seg- 

 mented ; two thoracic appendages Cypris. 



3. Order Copepoda. — Without shell ; five pairs of thoracic limbs ; 

 many forms parasitic and degenerate. 



I. Suborder Eucopepoda. — First thoracic segment only fused 

 with head ; abdomen cylindrical and segmented except in 

 highly degenerated forms Cyclops. 



4. Order Cirrhipedia. — Sessile or parasitic; segmentation indis- 

 tinct; six pairs of thoracic appendages; pass through Cypris 

 stage Barnacles. 



