56 ZOOLOGY 



are termed "tube-feet." In most star-fish and many sea- 

 urchins and sea-cucumbers the tips of these tube-feet are 

 terminated by circular discs, which act like suckers and enable 

 the animal to adhere to the surface over which it is passing, 

 and the animal literally walks on them. From the ring-canal 

 of the hydrocoele, in the interval between two of the radiating 

 canals, there arises a vertical canal termed the stone-canal 

 which terminates in a plate pierced with pores on the upper 

 surfaces of the animal. This plate is called the madreporite : 

 its pores are lined by ciliated cells, and its purpose is to 



FlG. 26. Upper view of common star-fish as an 

 example of Echinodermata ; a, anus ; madre- 

 porite ; sp, spines. 



introduce fresh supplies of water into the hydrocoele. Next 

 to the water- vascular system, the skeleton of Echinodermata 

 constitutes their most distinctive feature. In Sponges and in 

 some Ccelenterata the skeleton consists, as we have already 

 seen, of needles of calcareous or flinty matter embedded in the 

 jelly which intravenes between skin and digestive tube, but 

 in all the other phyla which we have so far discussed and 

 in most Ccelenterata the skeleton is of the nature of an 

 exudation or secretion from the external surface of the skin. 

 In Echinodermata, on the contrary, the skeleton consists of 

 calcareous plates embedded in the jelly beneath the skin, 



