NERITIC ZONE. 59 



The Nekton swim about actively and determine their 

 own movements. They are, as a rule, higher and more 

 complex types and show piano-symmetry {Sepia and Sagitta 

 are examples). 



ip) Neritic Zone. — The neritic zone extends from high 

 tide-mark down to about 500 fathoms. It includes only 

 the animals found at or near the bottom, and is a zone 

 containing a rich variety of forms. It can be divided 

 into two well-defined sub-zones — (i) the littoral, and (2) 

 the katantic. The littoral sub-zone extends between ex- 

 treme tide-marks. It has a variety of animals capable of 

 exposure to great extremes of temperature, and often to 

 lack of water. Exploration of rock-pools gives one a very 

 good idea of its inhabitants. There are numerous Echi?io- 

 dermata, Crustacea, Mollusca (especially gastropods), and 

 Annelida, whilst Coelenterata and fishes are common. 



We may divide neritic forms into two groups, according 

 to habits, as in the case of the pelagic. These are the 

 Nekton, as before, and the Benthos. 



The Nekton are swimmers which usually feed upon the 

 Benthos, less commonly upon each other. They present 

 many modifications similar to those of the pelagic Nekton, 

 but can usually be distinguished from them. For example, 

 a pelagic fish can usually be at once distinguished from a 

 neritic fish. 



The Benthos are a heterogeneous assemblage that live 

 on the sea-floor itself. We can discern the important group 

 of sedentary forms which corresponds to the Plankton of the 

 pelagic zone. They are fixed at one end to a foreign body, 

 and may have a tube or a burrow. They always show more 

 or less axial symmetry, and the higher types have a U-shaped 

 alimentary canal, mouth and anus opening away from the 

 point of fixation. They also frequently occur in colonies. 

 They belong to the Protozoa^ Porifera, Coelenterata, Echi- 

 noderjnata, Polyzoa, Brachiopoda, Crustacea, Mollusca, 

 Annelida and Tunicata {Sycandra, Obelia, and Actinia, 

 are examples). 



The second group creep or crawl over the surface of the 

 bottom, their weight being borne by it. These consist 

 principally of the creeping Mollusca and the crawling Crus- 

 tacea, These types are important, for they are the first to 



