XXVI TUBE-FEET 307 



as this, i.e., the division of the body into similar parts 

 radiating from a common centre, is characteristic of the 

 Echinodermata generally. 



In the centre of the disc on the ventral surface is a five- 

 sided depression, at the bottom of which is the large mouth 

 (Fig. 75 and Fig. 76, a, MtJi). From it radiate five grooves 



Fig. 75. — A Starfish, from the ventral aspect, showing the disc and 

 arms, the central mouth, and the numerous tube-feet. (From Parker 

 .and Haswell's Zoology, after Leuckart and Nitsche. ) 



called the ambulacral grooves, one along the ventral surface 

 of each arm (Fig. 76, a and b). In the living animal numerous 

 delicate semi-transparent cylinders, the tube-feet (Fig. 75 and 

 Fig. 76, T. F), are protruded from these grooves ; they are 

 very extensible and each ends in a sucker. It is by moving 

 these structures in various directions, protruding some and 

 withdrawing others, that the starfish is able to move along 



X 2 



