HOW DO STARFISHES OPEN OYSTERS ? 



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parallel to them. Then he set about opening and digesting the Natica. 

 This result clearly confirms the correctness of the above supposition. 



In the case of oysters the circumstances are different, in so far that, 

 under natural conditions, these animals are fixed to the ground, and are 

 also considerably larger than the other bivalves. If a starfish, wishing 

 to open an oyster, can find suitable points for fixing his arms on the 

 objects which lie around, it will give him no great trouble to pull his 

 victim apart. Should he not, however, find these, he must form a 

 mound exactly similar to that in Fig. 3. Physical considerations, how- 

 ever, show that under these circumstances, since he must support the 

 portion of the arms marked a — h in Fig. 3 on the oyster itself, there 

 will only be a prospect of success when this point of support of the 

 arms lies quite far towards the hinge, or even beyond it, so that the 



Fig. 5. 



arms can mutually assist each other. There must therefore always be 

 a definite size-relation between the oyster and the starfish ; and from 

 this it follows further that large oysters are relatively safe from the 

 attacks of starfishes, whilst small and medium sized ones are specially 

 liable to destruction. Perhaps some day an oyster fisherman will collect 

 evidence on this point. In relation to the matter I must, however, 

 remark that an oyster can only be regarded as successfully attacked 

 when it has been actually opened ; and a simple attempt on the part of 

 the starfish of itself proves nothing. In the figures which Collins gives 

 on Plate 165 (Figs. 1 and 2), the size-relation under discussion is clearly 

 seen; and I believe I am not mistaken when I imagine that I can see in 

 the positions of the starfishes in these figures the mound formation 

 which I have described. 



; In Fig. 5, which I give here from an observation of my own, the 

 starfish has already completed the work of opening ; and has, indeed, 

 already digested the greater part of its victim. There is nothing more 



