276 



HOW DO STAKFISHES OPEN OYSTERS? 



a section of Fig. 4, makes it at once evident that if the starfish intends 

 to open the bivalve by force, he can only do so after he has brought 

 himself and his prey into the positions there represented. I will not 

 here go further into physical considerations, but only remark that the 

 mound itself is extraordinarily rigid, and offers very great resistance to 

 any attempt to press it down. The starfish can now divide its tube-feet 

 in such a way that half of them are fixed to one valve of the shell, the 

 other half to the other ; and a pull in opposite directions can be exerted 

 upon the two valves. If the mound formation is adopted in order to 

 open the mussel in the manner indicated, a starfish which is prevented 

 from adopting such a position will not be able to succeed in opening a 

 free bivalve or gasteropod. I therefore made the following experiment: 

 I took a small vertical dish with glass sides, and, by means of a glass 



^m-''vAk\^a 



Fig. 



plate, separated off a compartment in which there was just depth enough 

 for a starfish to creep, but in which he could not form a mound. When 

 I had put a starfish, which had been prepared by previous fasting, into 

 this small compartment, I offered him a closed Natica, which he 

 immediately took. Now whereas, under ordinary conditions, provided 

 a long fight did not take place, a starfish would open a Natica in a 

 relatively short space of time,* this starfish wandered round the dish 

 for nearly a whole day, from morning till evening, with his victim — 

 which all the time remained closed — without managing to digest it. 



It was only towards evening, after many vain attempts, that by all 

 sorts of contortions of his arms he succeeded in forming a mound in a 

 (|uite unnatural Avay, namely, between the glass sides and in a position 



* There is no need to explain further that gasteropods are opened in exactly the same 

 way as bivalves ; some of the tube-feet of the starfish being fixed to the shell itself, whilst 

 others are fixed to the operculum. The gasteropods are brought into an exactly similar 

 position. 



