114 New York State Museum 



to such an extent that the mussels often are so numer- 

 ous as to occur growing upon one another to the depth 

 of several layers. The older forms gradually become 

 smothered beneath the ever-growing mass of younger 

 forms, and the mussels also actually poison one another. 



Factors dangerous to eggs and larvae. To a certain 

 extent the eggs and larvae of shore animals have to 

 contend with the same factors that make the struggle 

 so fearful for the adults of the species. Since the lar- 

 vae, often the eggs too, are part of the pelagic life of 

 the ocean they for a time escape some of it. For both 

 eggs and larvae there is danger of being stranded on 

 the shore or swept out to sea; danger from variation 

 in the environment due to tides, temperature changes 

 etc. ; danger from predaceous animals. For the eggs 

 there is the risk of dessication when the tide is out; 

 for the larvae, the risk of failure to obtain a foothold. 

 To offset these dangers the spawn of shore forms is 

 often attached and is given the additional protection 

 of some form of horny or gelatinous covering. There 

 is also a tendency to modify the chapters in the normal 

 life history according to need. Under certain condi- 

 tions the larval period may be prolonged ; under others 

 it may be condensed, as in forms living at high-tide 

 mark (for example, Periwinkles). The period of re- 

 production is often adapted to the seasons or coincides 

 with some particular phase of the moon which gives 

 the larvae the best chance of survival. It is obvious, 

 therefore, when eggs are liberated at random without 

 parental care, that they must be produced in large 

 numbers. In contrast to this, some of the shore fish 

 produce a small number of eggs because they have 

 acquired the habit of parental care. 



