252 



ZOOLOGY. 



Fifth Sub-Kingdom : ECHINODERMATA (Hedgehog- 

 Skinned Animals). 



The echinoderms are radially symmetrical, i.e. a 

 number of similar parts which together make up the 

 body are arranged around an axis (Fig. 145). These 

 animals are usually five-rayed. The mouth lies at 

 lower end of the axis of the body. In some echino- 

 derms the radial symmetry is indistinct, and at the 

 same time there is a more or less clear bilateral sym- 

 metry (p. 16). The dermal skeleton either consists 

 of numerous microscopic calcareous deposits, or of 

 firm plates; the skin often bears spines, hence the 



Fig. 115. — The Common Starfish (Asterias ruiens), seen on the right from above, 

 on the left from below. 



name of the group. Echinoderms move by means of 

 rows of tubular feet, which are connected with a 

 water-vascular system, and are protruded by means 

 of small bladders, connected with the same system, 

 which contract and force water into them. The form 

 of the body is apple-shaped to spherical (Regular 

 Sea Urchins), shield- or heart-shaped (Irregular Sea 



