198 COLLEGE ZOOLOGY 



taught individuals to use a certain arm in turning over. 

 One animal was trained in eighteen days (180 lessons), and 

 after an interval of seven days apparently " remembered " 

 which arm to use. Old individuals could not be trained as 

 readily as young specimens. 



Regeneration. — The starfish has remarkable powers of re- 

 generation. A single arm with part of the disc will regenerate 

 an entire body. If an arm is injured, it is usually cast off near 

 the base at the fourth or fifth ambulacral ossicle. This is 

 autotomy (see also pp. 117 and 155). 



Economic Importance. — Oyster beds are seriously affected 

 by starfishes. One starfish which was placed in a dish contain- 

 ing clams devoured over fifty of them in six days. Formerly 

 starfishes were taken, cut in two, and thrown back; this of 

 course only increased the number, since each piece regenerated 

 an entire animal. They are now often captured in a mop-like 

 tangle, to the threads of which the pedicellariae cling. They are 

 then thrown out on the shore above high-water mark and left 

 to die in the sun, or killed in hot water. 



2. Class I. Asteroidea — Starfishes 



Little need be said of the Asteroidea beyond what has been 

 stated above concerning one of the common species of the wide- 

 spread genus Asterias. The number of arms ranges from five 

 to more than forty, but aside from this diversity the chief dif- 

 ferences in shape among the starfishes are brought about by 

 the variations in the length and breadth of the arms and by 

 their lateral fusion. In some cases this adhesion has gone so 

 far as to result in a pentagonal form (Fig. 137). The skeleton 

 differs in structure in different species and is of importance in- 

 classification. 



The distinctive characteristics of the Asteroidea are as fol- 

 lows: Typically pentamerous; body commonly more or less 

 flattened; arms long or short, usually not sharply marked off 



