66 STARFISHES 



or retained within the pouches to undergo a direct and rapid 

 development. 



The mouth is surrounded by twenty tentacles, two on each 

 side of each oral angle, which is pierced for their passage. These 

 tentacles, which in the living Brittle Star are kept in constant 

 movement, are merely the modified tube-feet of the two first 

 arm-joints adapted to assist the entry of food into the mouth, and 

 the dispersal of the undigested residue, which is ejected from 

 the mouth, there being no second or anal opening to the stomach 

 as in the Common Starfish. The rays or arms of the Brittle 

 Stars are rather appendages of the body than actual portions of 

 it, as is the case with the Asteroidea. The greater part of each 

 ray is formed by a central bony axis, which is composed of suc- 

 cessive joints, and fills up almost the whole of the internal 

 cavity of the ray. In the Ophiurids the tube-feet have less to 

 do with locomotion than those of the Sea Urchins and Starfishes ; 

 their chief function is probably respiratory, locomotion being 

 effected by the undulating writhings of the slender arms. Both 

 the Brittle Stars and the Sand Stars are much more active than 

 the true Starfishes, the Brittle Stars keeping their rays in con- 

 stant wriggling movement. As their popular name denotes, they 

 have a habit of breaking their rays into fragments if alarmed, 

 roughly handled, or unwell, but the so readily discarded limbs 

 are always replaced by new ones after a shorter or longer 

 interval. 



The Sand Stars, as may be gathered from their popular name, 

 frequent sandy places on the floor of the sea, while the Brittle 

 Stars frequent the same situation and also the deep rock pools 

 low down on the shore towards low-tide mark. Many are abun- 

 dant in the neighbourhood of oyster beds and scallop banks, 

 possibly preying upon the " spat " at the season when it is ejected 

 from the parent bivalves, though their stomachs generally con- 

 tain minute shells of various foraminifera. 



While most Ophiuroidea have simple, undivided arms, the 

 members of the order Astrophytida x are exceptions to the rule, 

 their arms forking ten or twelve times, so as to form a regular net- 

 work of branches round the body disk. The rare and beautiful 

 Shetland Argus (Astrophyton scutatum Link), which may measure 



1 Greek, aster, star ; pheston, plant ; eidos, form. 



