PHYLUM ECHINODERMATA 197 



anus (a), and ccelom. After swimming about for a day or 

 two the brachiolaria attaches itself to a bit of seaweed and 

 five little arms grow out on one side, thus beginning the 

 development of the radially symmetrical starfish (I) . Most 

 of the brachiolaria collapses and in part disintegrates; 

 the young starfish soon starts out to hunt clams and oysters. 

 The growth of a starfish depends upon the amount of food 

 it is able to secure. Mead separated two which changed 

 from larvae at the same time and kept them under obser- 

 vation for thirty-nine days. One was fed, the other received 

 no food, and at the end of the period the former was 

 fifty times as large as the latter. A starfish may become 

 mature in less than a year; and, as each female lays several 

 thousands of eggs, the rate of increase is rapid. 



Although Asterias, like most starfishes, uses only sexual 

 methods of reproduction to produce new individuals, there 

 are some species which are able to multiply asexually. 

 Linckia, for example, breaks off the tips of its arms and such 

 fragments grow into complete new individuals. 



GENERAL REMARKS ON STARFISHES 



The starfish is a fairly successful representative of a 

 somewhat unprogressive biological principle. It has greatly 

 elaborated the radial type of symmetry, and represents, 

 with other echinoderms, the highest degree of speciali- 

 zation along such lines. That the radial arrangement of 

 body parts has some things in its favor cannot be disputed. 

 It is certainly advantageous to be able to move with equal 

 facility in any direction. It is desirable to be able to 

 take food or receive notice of danger on any side of the 

 body. 



But usually the disadvantages more than compensate for 

 the favorable features associated with radial symmetry. 

 As was pointed out in connection with the coelenterates, 

 it is hard for a radially symmetrical animal to correlate its 

 abilities for a particular end. A starfish has five arms 



