484 ELEMENTARY BIOLOGY 



DIVISION VII ECHINODERMATA (" spiny-skinned " animals). Radially 



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. 



DIVISION VIII ANNELIDA ("ringed" animals). Wormlike animals with 

 segmented bodies. The two most important classes are represented 

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



DIVISION IX ARTHROPODA ("jointed-legged"). The body segmented; 

 exoskeleton. 



Class i Myriapoda ( ft thousand-legged "). (Examples. Myriapods, 

 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, taran- 

 tula, mites, ticks.) 



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

 men ; antennae, compound eyes ; three pairs of legs ; one or two 

 pairs of wings (a few forms' wingless) ; air-breathers. A list of the 

 chief orders of this important class is given on page 487). 



DIVISION X. MOLLUSCA ("soft" animals). Unsegmented animals, most 



of them bearing shells. 

 Class i Gastropods (" belly-footed "). Having shells of a single piece. 



(Examples. Snails, slugs, periwinkle, whelk.) 

 Class 2 Pelecypoda (" hatchet-footed "). Bivalve (having shells of 



two valves). (Examples. Oysters, clams, piddock, scallop, mussel, 



shipworm.) 

 Class 3 Cephalopoda (" head-footed "). The foot partly surrounds the 



head and has a number of arms, or tentacles. (Examples. Octopus, 



cuttlefish, squid, nautilus.) 



DIVISION XI CORDATA. Animals having a notocord, or internal axial 

 basis for a skeleton. It is from this structure that the vertebral 

 column develops. There are a number of small animals which never 

 develop a true backbone, but which nevertheless have a structure 

 that suggests the beginning of such a column. These are included 



