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 plano-symmetry (Seda and Sagitta 
are examples). 
(0) Meritic 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—(r) 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 Zchino- 
dermata, Crustacea, Mollusca (especially gastropods), and 
Annelida, whilst Calenterata 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, Calenterata, Echi- 
nodermata, 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 
