1 6 VERRILL 



feet in Asteriidae, as compared with the more primitive two-rowed 

 condition, was probably due to the same causes. 



I am inclined to believe that the increase of rays has been due more 

 to the advantage gained in holding their food securely, and in 

 opening bivalves, than for holding to the rocks, though both go 

 together. The starfishes are the most predaceous of the echinoderms. 

 They feed largely on bivalve shells that cling firmly to the rocks, 

 like the oysters, and on others that must be pulled open by means of 

 their numerous muscular suckers, part of which must be used for 

 keeping in a suitable position and part for the opening of the oyster, 

 at the same time, so that the more arms and suckers they have, the 

 more rapidly and effectively they can secure their food, in case they 

 are feeding on large bivalves. 



We must admit that, so far as known, the five-rayed and six-rayed 

 individuals of a species appear to be equally well nourished and grow 

 to equal size. Also that the normally six-rayed species of Asterias 

 are commonly no larger, nor more robust, than the allied five-rayed 

 species, in the same environment. Even the four-rayed individuals 

 appear to be well nourished and of ordinary size. It is quite possible, 

 however, that they grow more slowly. It is known that the rate of 

 growth of starfishes is very rapid when food is abundant, but very 

 slow when food is scarce. No observations have been made on the 

 comparative rate of growth of six-rayed specimens. 



It must be remembered that the variation in the number of rays 

 is necessarily attended by extensive changes in the number, size, and 

 form of the skeletal plates of the body; also in the number of 

 ambulacral feet and water tubes, nerve ganglions, nerve cords, 

 stomach lobes, hepatic glands, and all other internal organs. 1 A six- 

 rayed specimen has twelve reproductive glands, instead of the ten 

 in its five-rayed competitor. If the number of ovules be propor- 

 tionately large, it would produce twenty per cent more young. 



So, likewise, it would have an additional stomach lobe and two 

 more hepatic glands. This would perhaps be of advantage in the 



1 This does not hold good in the cases of the multirayed species of Heliaster, 

 according to Clark. (The Genus Heliaster, 1907.) This genus has but five 

 stomach lobes and five pairs of gastric retractor muscles, whatever may be the 

 number of rays. Yet the added rays contribute to digestion in that group, 

 for each ray has a pair of hepatic glands and a pair of gonads. 



The term " hepatic gland " must not be understood as implying that these 

 glands correspond precisely with the liver of vertebrates, for it is known that 

 the digestive fluids of echinoderms are different chemically from those of the 

 latter. Some claim that these glands of starfishes are more like the pancreas 

 in function. 



