ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 483 



environments within the sea are, properly speaking, ecological phe- 

 nomena. A discussion of marine faunas (Fig. 302) in detail or of the 

 animal communities of the sea is impossible here but a few statements 

 may be made. The conditions met by animals living between high and 

 low tide marks vary so that these animals find life very difficult and have 

 to make many adjustments. Shifting beaches are unsuitable for many 

 forms which are abundant on permanent rocky shores. The fauna of 

 mud flats consists largely of burrowers. Pelagic forms show adaptations 

 making it possible for them to float and swim, and these affect the form 

 of the body, its structure, and the development of special organs. Such 

 animals, particularly if they are plankton forms, are characterized by 

 transparency and delicacy of color. The bottom forms of the sea exhibit 



/Sftore Fauna 

 ^ r In fer tidal Fauna Tide Level s-:^^ 



Liftoral Fauna Pelaqic Fauna < ,, , , 



^ - I Nekton 



Fig. 302. — Diagram to show distribution of animal life in the seas. The light is shown 

 fading out with increasing depth and ceasing entirely at 900 meters. 



many adjustments, including the presence of stalks to raise the sessile 

 forms above the mud, the development of long legs on the part of walking 

 forms, the production of few and large eggs, and other adjustments 

 permitting the parent to carry the young about and to protect them. 

 Even such deep-sea animals as echinoderms carry their offspring about 

 during development and until they are able to shift for themselves. 



The fauna of the sea in general has been divided into (1) the littoral 

 fauna, found along the shores, (2) the pelagic fauna, occupying the upper 

 levels away from the shores, and (3) the abyssal fauna, living in the depths 

 of the sea. The pelagic fauna is divided into the nekton, which includes 

 all the larger forms, that are able to control their own movements, and 

 the plankton, which includes the smaller forms, that are at the mercy of the 

 currents and the movements of the waves. The bottom forms in the 

 depths of the sea are called, collectively, the benthos. 



