144 GENERAL BIOLOGY 



Class 3. Cephalopoda. Mollusca with a well marked 

 head, a circle of arms bearing suckers around the 

 mouth, and well developed eyes. There is a heavy 

 muscular mantle fold; the nervous system is con- 

 centrated in the head. Squid, octopus. 

 PHYLUM ARTHROPODA. Segmented animals with a 

 firm external skeleton and jointed appendages. 



Class 1. Crustacea. Aquatic arthropods breathing by 

 means of gills, with two pairs of antennae and 

 .numerous pairs of biramous appendages on thorax 

 and abdomen. Crayfish, lobster, crab, water flea, 

 barnacles. 



Class 2. Myriapoda. Worm-like arthropods with numer- 

 ous similar segments bearing similar appendages. 

 One pair of antennae, breathe by means of tracheae. 

 Millipeds, centipedes. 



Class 3. Insecta. Arthropoda with the adults usually 

 bearing three pairs of legs and two pairs of wings; 

 body divided into head, thorax and abdomen; 

 single pair of antennas; breathe by tracheae. A 

 metamorphosis is common in the life history. Fly, 

 mosquito, beetle, grasshopper, bee. 



Class 4. Arachnida. Arthropoda without antennae, with 

 four pairs of legs and two pairs of mouth parts. 

 Body divided into cephalothorax and abdomen. 

 Breathe by means of trachea? and book lungs. 

 Spiders, mites, scorpions. 



PHYLUM VERTEBRATA. Animals with dorsal brain and 

 cord, enclosed in an unsegmented skull and a seg- 

 mented vertebral column. Red blood; usually 

 with two pairs of appendages. 



Class 1. Pisces. Aquatic vertebrates breathing by 

 means of gills; typically with two paired and other 

 unpaired fins. Fishes. 



