AQUATIC FAUNA. 57 
nature’s cradle, and in it still dwell numerous low types 
of animals, which indicate to us the structural plan of the 
earliest animals of our earth. ; 
If we take the four groups which stand at the base of 
animal creation, namely, the Protozoa, Porifera, Celenterata 
and L£chinodermata, we find that the sea has an entire 
monopoly of the Zchinodermata, a practical monopoly of 
the Porifera, and an immense preponderance of the other 
two. The same tale is told if we go on to the Polyzoa, 
Brachiopoda, and Annelida. It is only when we come 
to the Mollusca, Arthropoda and the Vertebrata, that a 
considerable number of terrestrial and erial types make 
their appearance, 
There is evidence for believing that the ocean was 
peopled with animal life for many ages before the dry land, 
hence it is not surprising that a number of nature’s lowest 
types still live on with little modification in the somewhat 
similar environment. 
Let us recollect that in the structural characters of 
animals, both young and old, we have attempted to distin- 
guish between the inherited and the acquired, the palin- 
genetic and the ccenogenetic. In a precisely similar way 
we may discern in marine fauna the palingenetic and the 
ccenogenetic inhabitants. In the case of the great majority 
of the lower phyla there is no reason to suppose that the sea 
has ever been forsaken. The marine Protozoa, Porifera, 
Calenterata and Echinodermata, have ever been marine, 
but there are a number of marine birds, some marine 
mammals (Cetacea and Strenia), and a few marine insects, 
which are evidently descended from terrestrial ancestors 
and have fallen from their high estate to once more rejoin 
their more lowly organised relations in the ocean. 
The marine fauna may be sub-divided into :— 
Pelagic zone. 
Neritic zone. 
Abysmal zone. 
(a) Pelagic Zone.—Of all the marine fauna, the pelagic 
zone includes probably the most primitive types. They 
consist of those animals which dwell at or near the surface of 
the ocean far away from land. They belong mainly to the 
