164 DISTRIBUTION. 



In the jilienoiucna connected with sul^uiarine vegetation, indeed, we shall find conditions 

 which exert an obvions inflnence on the distribution of animals. It is certain that in the range 

 of marine life a much more direct and intimate relation exists between depth and vegetation than 

 between depth and the presence of animal life. With the deeper regions vegetation has nothing 

 to do. We know that the higher forms of marine plant life entirely cease at a comparatively 

 slight depth, and even the lower forms disappear long before the deepest regions yet explored have 

 been reached, while no limit has yet been found to the extension downwards of living animals ; 

 even the BiatomacecB and other protophytal forms giving place in the deeper regions to their 

 protozoal representatives. 



Now, one of the most marked elements in determining the animal life of submarine zones 

 will naturally be sought for in the abundance or paucity of their vegetation ; for this affords food 

 to the vegetable feeders which in their turn supply the carnivorous tribes, while both find in its 

 more or less luxuriant development mechanical support and shelter. 



Though the Hydroida, less probably than many other groups of marine animals, are 

 dependent on the surrounding vegetation, it must yet be admitted that in the region of submarine 

 vegetation, and in that which stretches down beyond it, we have thus the limits of two natural 

 zones of depth, and there can be no doubt that these present us with two sets of conditions which 

 go far to determine the bathymetrical distribution of animals. 



But in order to express with sufficient accuracy the distribution of animals in depth, 

 a more minute division is needed, and it will be found convenient to divide the entire 

 depth into several definite zones. Of these the higher ones will be each characterised not 

 only by its faima, but by the form of vegetation which is special to it ; wdiile below we have a 

 vast plantless region stretching downwards into depths which until lately were regarded as 

 unfathomable. 



The zones which lie between high water mark and the lowest level of spring tides, and con- 

 stitute the Literal and upper part of the Laminarian zone of Forbes, are, at least along the tidal 

 shores of our own latitudes, where they are loaded with a profuse marine vegetation, very rich in 

 hydroid life. In this region two distinct physical habitats must be distinguished, each exerting its 

 special influence on literal life, and each characterised by a more or less definite hydroid 

 fauna. 



One of these consists of the ground which during the ebb becomes exposed to the air 

 retaining only so much moisture as may be prevented from evaporating by the clothing of sea- 

 weed and the projecting ridges of rock. The other is formed l)y the rock pools, — reservoirs of 

 water of greater or less extent which are left behind by the retreat of the tide. 



Again, in the deeper regions the physical condition of the bottom, whether rocky or covered 

 with large stones, or sandy or shelly, or overspread wiih ooze or mud, will exercise an important 

 influence on the distribution of the Hydroida and of other marine animals. 



Among the positive facts which the scientific exploration of the deep sea has established with 

 regard to the distribution of the Hydroida, one of the most important is that the range in depth 

 of hydroid life is not surpassed by the known range of any other group of the animal kingdom. 

 For while some species occur rooted to the rocks and seaweeds close to high-water mark — not to 

 mention the pelagic planoblasts whose life is spent on the very surface of the open sea amid all 

 the influences of the atmosphere and of the light and heat of the sun — there is evidence of the 

 existence of others at a depth of nearly three miles from the surface, the " Porcupine" explorers 



