No. 634] UTILIZATION OF ECHINODERMS 425 



podia helianthoides ) . As a matter of fact, bays that have 

 none or very few Pycnopodia may have a large number 

 of Polynices, and bays that are well populated with this 

 starfish have remarkably few Polynices present. When 

 I later experimented on the sensitivity of Polynices to 

 Pycnopodia (Agersborg, 1918) I found in all instances, 

 that when the slug came into contact with the star, it 

 withdrew its foot at once. The monstrous foot, though it 

 seemed impossible that it could be withdrawn within the 

 shell, was very quickly covered thereby. Upon with- 

 drawing the foot in a hurry, as it does when in contact 

 with Pycnopodia, the periphery of the foot, which is per- 

 forated, throws a spray like a garden sprinkler with the 

 holes in the spray-disk plugged except those around the 

 periphery. No matter how much larger the animal is than 

 its shell, when all the water is squeezed out of the foot, 

 the former can be completely covered by the latter. In 

 such a condition, however, Polynices can not live very 

 long. It is itself easily exhausted when completely shut 

 up within its shell. If it is not allowed to take in fresh 

 water supply when it comes out to breathe it soon relaxes, 

 an easy prey to the gluttonous Pycnopodia. In fact, when 

 leaving Polynices with Pycnopodia in an aquarium, two 

 of the former were killed and eaten by the latter within 

 three days, leaving the shells and opercula. 



The absence of Polynices where Pycnopodia abounds, 

 together with the facts observed when keeping the two in 

 the same aquarium, seems to indicate definitely that 

 Pycnopodia preys on Polynices. As mentioned above, 

 Polynices is a nuisance to oyster growers, even if it does 

 not feed on oysters, for it destroys the oyster beds by 

 burrowing in them; primarily Pycnopodia is a gastero- 

 pod feeder, and though it is quite omnivorous and may 

 feed on anything it happens to encounter, it is not known 

 whether it feeds on oysters. The question then is : might 

 not Pycnopodia be used as a check against Polynices in 

 the oyster beds? This could easily be tested out experi- 

 mentally : Pycnopodia could be placed on oyster beds to 



