STARFISH. 229 



and mud, the Holothiirians in particular practising this 

 worm-like mode of nutrition. 



Most Echinode7-ms have to a remarkable extent the power 

 of casting off and regenerating portions of their body. This 

 power is frequently reckoned as one of their means of defence, 

 but they often mutilate themselves merely as a consequence of 

 unfavourable conditions of life. The self- inutilation, or 

 autotomy, as it is called, seems to be entirely a reflex action, 

 not voluntary . 



The peculiar water vascular system attains great develop- 

 ment, and has usually respiratory or locomotor functions. It 

 is possible that i?i some cases it may also function as an organ 

 of excretion. Well-defined excretory organs are conspicuously 

 rare. Soluble waste products seem generally to diffuse out 

 into the water, while the insoluble are here, as in sea squirts, 

 stored up in the tissues in the form of granular masses. 



The Holothiirians are in form nearest to the supposed worm-like 

 ancestor, and are perhaps primitive forms, which do not lead up to any 

 of the other classes. From primitive unspecialised Cystoids, the 

 Echinozoa, and Pelmatozoa have perhaps taken origin. Of the 

 Echinozoa the Asteroidea and Ophiuroidea are very closely related, and 

 seem to be connected by fossil forms. 



In our survey of the group it is more convenient to begin with the 

 familiar starfishes than with the more primitive forms. The general 

 characters of each class may be read from the synoptic table at the end. 



Class Asteroidea. Starfish. 



The description applies especially to the common five- 

 rayed starfish {Asterias or Asteracanthion rubens). It is 

 often seen in shore pools exposed at low water, but its 

 haunts are on the floor of the sea at greater depths. There 

 it moves about sluggishly in any direction by means of its 

 tube feet. 



Form. — Each of the five arms bears a deep ventral groove 

 in which the tube feet are lodged. The mouth is in the 

 middle of the ventral surface, the food canal ends about 

 the centre of the dorsal disc. With this flat, five-rayed 

 form, the 11-13 rayed sun star {Solaster), the pincushion- 

 like Goniaster, and the flat pentagonal Palmipes, should be 

 contrasted. 



