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 (" thousand-legged "). {Examples. Myriapods, 

 centipede.) 



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

 breathers ; antennae. {Exafiipks. 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). {Exatnples. 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 



