THE CRAYFISH AND ARTHROPODS IN GENERAL 225 



2. ARTHROPODS IN GENERAL 



All the segmented animals bearing jointed 

 appendages belonging to the Phylum Arthro- 

 poda may be grouped in five classes : 



Class I. Crustacea (Crayfish, Crabs, Bar- 

 nacles, Water-fleas, etc.). 



Class II. Onychophora (Peripatus). 



Class III. Myriapoda (Centipedes and 

 Millipedes). 



Class IV. Insecta (Insects). 



Class V. Arachnida (Spiders, Mites, Scor- 

 pions, King crabs, etc.). 



These five classes are often divided for 

 convenience into two large groups, the Bran- 

 chiata containing Class I, and the Tra- 

 cheata, Classes II-V. Members of the former 

 division are mainly aquatic and breathe by 

 means of gills. The Tracheata are in most 

 cases terrestrial, and obtain oxygen from air 

 taken into a complex system of tubules, called 

 tracheae, which ramify to all parts of the 

 body. 



Arthropods are supposed to be closely 

 related to Annelids. The members of both 

 phyla are segmented, bilaterally symmetrical, 

 and triploblastic, with a dorsal brain in the 

 head and a ventral nerve cord, a dorsal heart, 

 and an external chitinous covering. The 

 following differences may be pointed out : 

 Annelids possess a large number of similar 

 segments; Arthropods, in most cases, a 

 limited number of much modified segments: 

 the former have segmentally arranged 

 nephridia; the excretory organs of the latter, 



FIG. 119. Ventral 

 view of male Cy- 

 clops, i, anten- 

 nule ; 2, anten- 

 na ; 3, mandible ; 



4, ist maxilla; 



5, two halves 

 of 2d maxilla; 

 6-p, i s t - 4 t h 

 thoracic limbs ; 

 10, eye; n, bris- 

 tles near male 

 genital opening; 

 12, caudal fork ; 

 73, mouth ; 14, 

 copula connect- 

 ing pairs of 

 limbs. (From 

 Shipley and 

 MacBride.) 



