THE ECHINODERMS 



155 



in a vessel with plenty of sand for a few days, with the object 

 especially of studying the method of burrowing, which is effected 

 by certain curious flattened spines, and the method of feeding. 

 The heart-urchin feeds on minute particles in the sand, and there 

 is a very elaborate arrangement of spines, grooves and tube-feet 

 whose object is to sweep the particles into the toothless mouth. 



A considerable number of starfishes occur in the rock pools, 

 the number and kinds varying with the locality. It is sufficient 

 for our purpose to note the 

 distinction between the five- 

 fingered forms, of which 

 Asterias rubens, the common 

 starfish, is perhaps the 

 commonest, and the forms 

 with more than five arms 

 or fingers, of which the sun- 

 star (Solaster papposus) is an 

 example. In either note the 

 central disc which passes 

 insensibly into the tapering 

 arms. On the dorsal or 

 upper surface of the disc 

 notice the conspicuous mad- 

 reporite, or perforated plate 

 which allows water to enter the water- vascular system. The under 

 surface is more interesting. In the centre notice the mouth, which 

 has no lantern like that of the urchin. The under surface of each 

 arm is deeply grooved, and in the groove lie the tube-feet, which 

 are capable of considerable elongation. Each ends in a sucker, 

 and by fixing these suckers the starfish can crawl up a perfectly 

 perpendicular surface just as a fly can. The delicate tube-feet 

 are protected by spines at the sides, and if destroyed can be 

 readily re-grown ; if a sea-urchin, for example, be torn suddenly 

 from the rock to which it is clinging, it will be often found to 

 leave some of its tube-feet behind it, but this is not of great 

 importance, for the feet quickly grow again. 



We have noted above that normally the common star has 

 five arms or rays, but every now and again one may come across 



FIG. 76. The sun-star (Solaster). Note the 

 madreporite to the left of the central disc. 



