62 



INTRODUCTION TO ZOOLOGY 



31, jj); under a lens they can be seen snapping together 

 and opening again, and it is thought their function may be to 

 keep the body clear of the small particles of foreign matter 

 that might so easily catch and accumulate between the spines. 

 To one side of the central disc, between the bases of two 

 arms, may be seen a small calcareous perforated plate known 

 as the MadreporUe (Fig. 31, md), the function of which is 

 explained later (see p. 65). 



On turning the starfish over, it will be seen 

 that a deep groove runs from the tip of each 

 arm to the centre of the disc where the large 

 mouth is situated. Each groove contains two double rows 



Ventral 

 Surface. 



Fig. 31. 



-Section through the Disc and one of the Arms of a Starfish. 

 (Diagrammatic. ) 



mtli, Mouth ; a, anus ; p, pedicellariae ; tf, tube-feet ; b, respiratory vesicles ; sc, stone 

 canal ; md, madreporite ; g, generative organ ; I, one branch of digestive sac or 

 "liver-gland"; e, eye-spot. 



of transparent finger-like " tube-feet," the tips of which form 

 suckers, that enable a starfish to adhere very closely to a rock, 

 and which are also used in its characteristic slow motion 

 over a- vertical or horizontal surface. By means of them the 

 starfish can turn itself over if placed back downwards. 



These tube-feet vary greatly in length and shape, for they 

 can be extended or contracted at will, owing to the action of 

 the peculiar water-vascular system (see p. 65). 



When the creature wishes to move in any 



special direction, the tube-feet of the arm on that 



side all extend and attach themselves to some object by the 



