SOME ASPECTS OF ZOOLOGY 39 



genera and species. All have a similar mode of nutrition. 

 They feed on microscopic animals filtered out of the water by 

 the sieve-like apparatus of their gills. Take as a familiar 

 example the common oyster, the life-conditions of which I 

 studied in detail some years ago. It has many enemies, chief 

 among them the dog-whelk and the starfish, against which 

 its stout shell is only an insecure protection, but how far 

 can it be said to compete with its own kind ? Certainly 

 oyster cannot be said to enter into vigorous competition 

 with oyster, for where enemies are kept in check, as in oyster- 

 beds, the oysters occupy the ground as thickly as they can 

 lie, and flourish and grow fat in this crowded condition, pro- 

 vided, of course, that food-bearing currents are present. Nor 

 do they enter into competition with other lamellibranchs, 

 e.g. with mussels, which affect other localities where they can 

 cling to rocks or piles by their byssus, nor yet with pectens, 

 or with cockles or Tapes, Venus, Cy there or other forms with 

 siphons which bury themselves in sand or mud. The only 

 " competitors " that I know of are Anomia and the gastropod 

 Crepidula. The former, I think, is not so much a competitor 

 as objectionable to the oyster-culturist because it spoils the 

 look of the shells. The latter does appear to be detrimental 

 to oyster-beds when, as in Essex, it increases in such numbers 

 as to smother and crowd out the oysters. It is an importation 

 from America, and an instance of the commonly observed fact 

 that a foreigner tends to upset the equilibrium established 

 among indigenous inhabitants. 



In general, lamellibranchs occupy somewhat different 

 localities in the marine territory, and are so adjusted to their 

 particular conditions that they do not interfere with one 

 another. But they are preyed upon by all sorts of other 

 animals and display all sorts of adaptive structural devices to 

 protect themselves from being eaten. These appear to be 

 only partially successful, for there is little doubt that the main 

 protection of any lamellibranch species against extinction by 

 its enemies is its immense fecundity. 



Without giving any further examples, which might be 

 multiplied manifold, I submit that too much stress has been 



