166 DISTRIBUTION. 



The great majoiity of the Hydroida, however, belong to umch shallower water, where they 

 are chiefly distributed through the great belt of submarine vegetation already referred to. This 

 belt, as stated above, extends downwards through a region divisible into definite zones of depth. 

 In the systematic part of the present work I have given, so far as it was possible to determine 

 them, the bathymctrical areas occupied by the various species described. These I have endeavoured 

 to express in bathymctrical zones based on those of Forbes — a method which will apply sufficiently 

 well to the Atlantic shores of Europe and of North America. It is less applicable to the 

 Mediterranean, where the Litoral and Lammarian zones, which take so very important a part in 

 the hydroid distribution of our own shores, can scarcely be said to exist,^ while from other 

 parts of the world the data we have received are so scanty that scarcely anything can be 

 asserted regarding the depths inhabited by such few hydroid species as have been found there. 



Batliymetrical zones and their characteristic hydroids. — In order to convey some idea of the 

 leading facts connected with the distribution of the Hydroida in depth, it may be well to take a 

 glance at the most striking features presented by each of those zones which, in the systematic por- 

 tion of the present work, will be referred to in recording the batliymetrical range of the species 

 there desci'ibed. They are, with some modifications, identical with the zones of depth, as originally 

 laid down by Oersted for the Danish coast, and afterwards extended to greater depths and otherwise 

 developed by Forbes. Our survey will embrace their leading physical and botanical characters 

 and the most striking features of their hydroid faunas as presented round the shores of the British 

 Islands. From what we have already said, however, we must avoid giving too much weight to 

 the influence of these zones on distribution, for few species are absolutely confined within the 

 limits of any one of them. 



The depth-regions which we shall here distinguish are six in number, namely, 

 I. The Surface Zone. II. The Litoral Zone. III. The Laminarian Zone. IV. The 

 Coralline Zone. V. The Deepwater Zone. VI. The Abyssal Zone. 



I. The Surface Zone. — This, though an exceedingly important depth-region, has been very 

 generally ignored as a special zone in the bathymctrical range of marine life. It is formed by the 

 stratum of water which reaches from the surface of the sea to a depth of two or three feet, and has 

 nothing to do with the subjacent ground. It is exposed to the direct action of the sun and of 

 the atmosphere, and is, more than any of the other zones, under the influence of latitude and 



compared in depth to those of the " Porcupine" explorers were attempted. Some of the hydroids 

 obtained have been examined by Mr. de Pourtales, wlio describes five new species, namely, an Antennu- 

 laria, three species of Halecium, and a Tubularia ; none of these, however, came from a depth greater 

 than 270 fathoms. (See " Contributions to the Fauna of the Gulf Stream at Great Depths." By 

 L. F. de Pourtales, Assist. U.S. Coast Survey, Bui. IMus. Comp. Zoology, at Harvard College.) 



I am myself engaged on the remainder of the hydroids collected duiing this important expedition, 

 and hope to be soon able to make known the results of my examination. 



* It may be supposed that the species which inhabit the region between the high and low water 

 lines on coasts where there is a well-marked tide range are fully represented on coasts where the tide 

 range is but slight, with this difference only, that in the former case they are spread over a wide belt, 

 in the latter condensed witliin a narrow one. Experience, however, shows that this is not the case, 

 and that shores with a narrow tide range have but a poor litoral fauna. 



