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^ Forifera, Coelenterata 

 and Echinoderttiata, we find that the sea has an entire 

 monopoly of the Echinodermata, 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 aerial 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 Httle 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 paHn- 

 genetic and the coenogenetic. In a precisely similar way 

 we may discern in marine fauna the palingenetic and the 

 coenogenetic 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, Port/era, 

 Ccelenterata and Echinoder7nata^ have ever been marine, 

 but there are a number of marine birds, some marine 

 mammals [Cetacea and Sirenia), and a few marine msects, 

 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 



