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THE AMERICAN NATURALIST [Vol. XLIII 



These same adaptations would also be useful in ena- 

 bling the creatures to securely hold their prey, especially 

 while, at the same time, holding fast to the rocks. 



I am inclined to believe that the increase of rays has 

 been due more to the advantages gained in holding the 

 food securely than for holding to the rocks, though both 

 go together. The starfishes are the most predacious of 

 the echinoderms. Although they feed largely on gastro- 

 pods and bivalve mollusks, they also feed on each other, 

 and on large holothurians, echini, and other relatively 

 large creatures. 



However, we must admit that, so far as now 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 

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

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

 individuals, including the four-rayed sea-urchin (Arba- 

 cia), mentioned above, appear to be well fed and of 

 average size. 



Some, at least, of the many-rayed Brisingida? use their 

 slender rays for clinging to the deep-sea Gorgonians. 1 

 have observed that the Odinia amrricana Ver. thus clings 

 to the branches of Paragorgia arborea with its twenty 

 long rays. In such cases numerous rays would be ad- 

 vantageous. 



It must be borne in mind that the variation in the 

 number of rays is necessarily attended by extensive 

 changes in the number, size and form of all the skeletal 

 plates; also in the number of ambulacral feet and water 

 tubes, nerve ganglions, nerve cords, stomach lobes, he- 

 patic glands and all other internal organs. A six-rayed 

 specimen has twelve reproductive glands, instead of the 

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

 be proportionately large, it would produce twenty per 

 cent, more young. So, likewise, it would have an addi- 

 tional stomach-lobe and two more hepatic glands. This 

 would, perhaps, be of considerable advantage in the diges- 

 tion of food and cause more rapid growth. We know that 



