CHAPTER IV. 



FOURTH BRANCH OF ANIMALS. 



STAR-FISHES, SEA-URCHINS, ETC. (Echinodermatd). 



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 



FIG. 30. Section of one ray of star-fish. A, central body ; S, stomach ; m, 

 mouth ; //, madreporic plate ; r, ring about the mouth ; B, ray ; sp, 

 spines set in leathery coat ; c, pedicellariae ; e, eye ; /, f, tubular feet ; 

 v, v, vesicles for supplying feet with water ; /", 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- 

 4 



