HOW DO STARFISHES OPEN OYSTERS ? 271 



protruding foot is squeezed, after half to three-quarters of a minute 

 it becomes paralysed, and can make no more use of the adductor 

 muscles. Apgar believes that the musk-rat {Fiber zihdhicus) takes 

 advantage of this fact in order to get at the soft parts of the mussel. 

 In the case of the oyster, however, anything of this kind does not 

 apply, for it cannot be brought into an unaccustomed attitude, nor has 

 it any foot to protrude and be squeezed. This could, however, happen 

 with other bivalves which are not fixed and which possess a pro- 

 truding foot ; for, whilst being eaten, these are constantly placed by 

 the starfish in a position quite the reverse of the normal one ; namely, 

 with the hinge below, and the gape above. I have tried experiments 

 on the point with Venus verrueosa, but have failed to notice any 

 hypnotic or paralytic effect. I have made a Venus stand for many 

 hours on the hinge, and have afterwards found exactly the same re- 

 sistance to forcible opening as at other times. Since, however, the 

 starfish can effect the opening in from fifteen to twenty minutes, the 

 possibility of hypnotic efiect is precluded. 



4. The starfish might make an opening in the shell with the help of a 

 boring apparatus or an aeicl. — No boring apparatus is possessed by 

 Asterias, and the holes which one often finds in oyster shells are due 

 to gasteropods, and not to starfish, although they have sometimes, in 

 error, been ascribed to the latter; e.g., by Ball and Forbes. I have 

 neither been able to find holes in the shell of a Venus which has been 

 devoured, nor an acid reaction in the everted stomach. It is, however, 

 a difficult thing in sea water, which is slightly alkaline, to demonstrate 

 an acid with litmus paper; but when we recollect that the opening 

 is effected in so short a time, the acid would necessarily require to be 

 very strong, and should be capable of demonstration even under such 

 unfavourable conditions. One does often find regular holes on the 

 shell-margin of oysters which have been eaten, but, as we shall see 

 presently, these are produced, not by boring, but by breaking. 



We come now to the possibility — 



5. That the starfish pours a poison over, or, rather, within the shell of 

 its victim, luhercby the muscular force of the latter is enfeebled, and the 

 shell op>ened. — In itself this is not unlikely, and I was at first of opinion 

 that this was, in fact, the method by which the opening was effected, 

 for it is known that many animals maim their victims by poison, 

 derived generally from the salivary glands, before devouring them. 

 However, even this power would not be of much use to the starfish. 

 As already mentioned, a Venus, for instance, squeezes its shell so 

 tightly together that one could almost speak of its being hermetically 

 closed. A poison poured over the shell could not penetrate, but M'ould 

 flow off without effect. In this case also it would be first necessary to 



NEW SERIES. — VOL. IV. NO. 3. U 



