CHAPTER IV 



ECHINODERMATA 



WE will now consider the star-fishes and their relations 

 the group Echinodermata " spine -skinned." 



They are thus defined : Animals of radial symmetry, 

 usually in five rays, with a firm leathery coating, protected 

 with spines or plates of carbonate of lime, or else (in one 

 division) with a shelly test studded with (usually) sharp 

 spines. 



They have a perfect digestive system, an organ acting as 

 heart, a nerve system, and a reproductive system, the 

 male and female elements situate in separate individuals. 

 (This last part of the definition applies to the species on 

 our coasts. There are one or two exceptions in exotic 

 species.) 



Locomotion is performed by " tube feet " (Ambulacra), 

 which are hollow, protrusible processes, exerted or re- 

 tracted by water pressure, operated by a curious arrange- 

 ment within the animal, termed the " water vascular 

 system." 



At the extremity of the tube feet or Ambulacra are little 

 suckers, which give them a hold upon the surface being 

 travelled over. 



The tube feet are protruded through openings in the skin 

 or test, which holes are in rows on the sides of radial de- 

 pressions which are termed " Ambulacral grooves " (see 

 Figs. 24 and 25. 



The reproduction is by eggs, these giving rise to free- 



