The Classification of Animals. 197 



Crustacea are mainly aquatic. They may be divided into four 

 classes. 



1. Prototracheata. — This group includes only the Myria- 

 opod-like Peripatus, which, while possessing the characteristic 

 jointed appendages of the Arthropoda, and the tracheae of the 

 present division of Arthropods, agrees with the worms in having 

 metameric nephridia. It is a caterpillar-like creature found in 

 tropical regions in many parts of the world. 



2. Myriapoda. — This group includes the centipedes and 

 millipedes. They have trachese, like Peripatus, but the body is 

 not distinguishable into thorax and abdomen. In the milli- 

 pedes each segment except the first few is provided with two 

 pairs of appendages ; in the centipedes each segment has only 

 one pair. 



3. Insecta. — Insects have a distinct head, thorax, and abdo- 

 men ; the latter has no appendages except a few doubtful 

 outgrowths, which may possibly belong to this category 

 (cf. p. 50). The animals breathe by tracheae. There is an 

 immense variety of insects which are grouped into the orders 

 Aptera (silver fish), a group of very primitive insects, without 

 metamorphosis and without wings; Orthoptera (cockroach, 

 earwig, grasshopper), with biting mouth-parts and but little 

 metamorphosis ; Neuroptera (dragon-flies, May-flies), with four 

 membranous wings and mouth-parts of the biting type, and 

 usually a pronounced metamorphosis ; Lepidoptera (butterflies, 

 moths), with a complete metamorphosis, four partly or entirely 

 scaly wings, sucking mouth (proboscis) ; Coleoptera (beetles, 

 with biting mouth-parts, complete metamorphosis, and four 

 wings, of which the first pair, as in the Orthoptera, are horny and 

 not used for flight ; Hemiptera (bugs), with incomplete meta- 

 morphosis, piercing mouth-parts ; Diptera (two-winged flies, 

 e.g. house-fly), with only one pair of fully developed wings, 

 piercing mouth-parts and complete metamorphosis; Hyme- 

 noptera (bees, wasps, ants), with four membranous wings, 

 biting mouth, complete metamorphosis. 



4. Arachnida. — ^This large group comprises Arthropods, 

 which breathe by tracheae, as in insects, or by lung-books, which 

 may be external, as in the king crab, or internal, as in the 



