CRAYFISH 395 



Division A. Prototracheata. The primitive form with some 

 arthropod characteristics and certain annelid features, such as 

 nephridia. 



Class II. Onychophora, Peripatus, the wormlike arthropod. 



Division B. Antennata. More highly specialized forms with one 

 pair of antennae. 



Class III. Myriapoda, centipedes and millepedes (thousand legs) 

 having one or two pairs of appendages on each segment. 



Class IV. Insecta, beetles, bees, locusts, etc., all with three pairs 

 of thoracic appendages and most of them with wings. 



Division C. Arachnoidea (a ak noi' de a, spiderlike). A group 

 without antennae but with tracheae, book lungs or book gills, and 

 four pairs of thoracic appendages. 



Fig-. 159. — Cambarus clarkii, the swamp crayfish, couimon through southern United 



States. 



Class V. Arachnida, spider, mite scorpion, king crab, etc. 



This summary of the classification of the phylum has been in- 

 cluded here in order that the student may realize its magnitude and 

 the great variety of animals included. A more complete outline 

 may be found in the chapter on animal classification. The number 

 of species described under the phylum is approximately one-half 

 million, and there are large numbers still undescribed and unnamed. 



Since the crayfish represents a relatively simple type of arthropod 

 and is so generally well known, it serves ideally as a representative 

 species for a more detailed study. The two genera Cambarus and 

 Potomohms are commonly found in the streams of North America. 

 The former is distributed east of the Rocky Mountains and the 

 latter on the Pacific slope. 



