524 TAXONOMY 



or in all three places. Most of them undergo a metamorphosis. 

 There are five classes : 



Class I — Onychophora. Primitive air-breathing arthropods 

 with tracheae and nephridia. Peripatus capensia. 



Class II — Crustacea. Breathe by means of gills. The head 

 and thorax are fused ; two pairs of antennae. They have a 

 " crusty " exoskeleton, strengthened with lime. Examples, 

 crabs and lobsters. 



Class III — Myriapoda (numberless legs). Have long bodies 

 with many segments and many paired jointed appendages ; 

 breathe by tracheae. Centipedes and millipeds are examples. 



Class IV — Arachnida (Gr., arachne — spider). This group 

 has no antennae, four pairs of legs, and a pair of clawlike 

 appendages on each side of the mouth. Head and thorax com- 

 bined as in Crustacea. Breathe by book-gills, book-lungs, or 

 tracheae. The spiders, daddy-long-legs, scorpions, mites, and 

 ticks are in this class. 



Class V — Insecta. The largest class of animals. Body seg- 

 mented ; three body regions, head, thorax, and abdomen ; 

 three pairs of jointed legs ; usually compound eyes ; breathe 

 through tracheae or air tubes. Insects. 



Phylum X. — CHORD ATA (Lat., chorda — cord). Animals 

 having a skeletal axis, gill slits in embryo or adult, and a nerve 

 cord dorsal to the alimentary canal. This phylum is divided into 

 four subphyla, one of which is the Vertebrata in which a nerve 

 cord is protected by a segmented, bony spinal column. These 

 vertebrates are divided into seven classes. 



Class I — Cyclostomata. Eel-like vertebrates with round 

 mouths and without functional jaws, without scales and fins. 

 Lampreys and hagfishes. 



Class II — Elasmobranchii. Fishlike vertebrates without a 

 bladder, with jaws, and with a cartilaginous skeleton. Sharks, 

 rays, and skates. 



