82 



BIOLOGY AND HUMAN LIFE 



Fig. 47. The pill bug 



When suddenly disturbed, this animal curls up, thus reducing its exposed surface and 

 concealing its most delicate and sensitive parts 



BRANCH VI— WHEELWORMS (Trochelminthes). The Rotifera, or 

 wheel animalcules. Mostly microscopic. 



BRANCH VII— ECHINODERMATA (''spiny-skinned" animals). Ra- 

 dially symmetrical, all marine. 

 Class i — Asteroidea. Starfish. 

 Class 2 — Ophiuroidea. Brittle stars. 

 Class 3 — Echinoidea. Sea urchins. 

 Class 4 — Holothuroidea. Sea cucumbers. 

 Class 5 — Crinoidea. Sea lilies. 



BRANCH VIII— ANNELIDA (''ringed" animals). Wormlike animals 

 with segmented bodies. The two most important classes are repre- 

 sented by earthworms, sandworms, etc. and by the leeches. 



BRANCH IX— ARTHROPODA (" jointed-legged"). The body seg- 

 mented ; exoskeleton. 



Class i — Myriapoda ("thousand-legged"). {Examples. Myria- 

 pods, centipede.) 



Class 2 — Crustacea ("crusty" shells). Head and thorax fused; 

 water-breathers ; antennae. {Examples. Lobster, crayfish, 

 crab, shrimp, barnacle, sow bug.) 



Class 3 — Arachnida (spider family). Four pairs of legs; air- 

 breathers ; no antennae. {Examples. Scorpions, spiders, 

 daddy longlegs, tarantula, mites, ticks.) 



Class 4 — Insecta. Segmented bodies; distinct head, thorax, and 

 abdomen ; antennae, compound eyes ; three pairs of legs ; 

 one or two pairs of wings (a few forms wingless) ; air- 

 breathers. The chief orders of this important class are 

 as follows : 

 I. Aptera ("without wings"). The most primitive insects now 

 living. {Examples. Silverfish and springtail.) 



