CHAPTER IV. 
FOURTH BRANCH OF ANIMALS. 
STAR-FISHES, SEA-URCHINS, Etc. (Echinodermata). 
General Characteristics—Marine radiate animals, hav- 
ing a calcareous skeleton made up of many plates. They 
possess a nervous system, and are distinguished, especially 
from the former groups, in having a tube-like digestive 
canal, distinct and separate from the cavity of the body. 
Skeleton.—By making a vertical section of the common 
star-fish (Fig. 30), we find that the skeleton is made up 
Fic. 30.—Section of one ray of star-fish. 4, central body; S, stomach; m, 
mouth; 4, madreporic plate; 7, ring about the mouth; B, ray; sf, 
spines set in leathery coat; ¢, pedicellarie; ¢, eye; 7,2, tubular feet ; 
uv, v, vesicles for supplying feet with water ; 2, liver. 
of calcareous plates, connected by a leathery integument, 
and covered by the skin, having spaces between them to 
allow the ingress of water. The plates increase by addi- 
tions to their edges ; thus their shape is preserved. The 
spines, or defensive organs, sp (Fig. 30), work on a ball- 
and-socket plan, and among them occur worm-like append- 
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