210 THE INHABITANTS OF THE SEA. 
The composition of the waters has also a most important 
effect on the distribution of aquatic animals, a3 the degree of 
saltness or freshness determines the presence or absence of 
numerous forms of both fishes and invertebrate animals; and 
last, not least, the influence of depth, in which pressure and 
the diminution of light are doubtless important elements, is 
everywhere manifest over the ocean, “for everywhere we find 
creatures, whether animal or vegetable, distributed in successive 
belts or regions, from high-water mark down to the deepest 
abysses from which living beings have been drawn up. Peculiar 
types inhabit each of the zones, and are confined within their 
destined limits, whilst others are common to two or more, and 
not a few appear capable of braving all bathymetrical conditions. 
Nevertheless, so marked is the appearance of the’ inhabitants of 
any given region of depth, that the sight of a sufficient assem- 
blage of them from any one locality will enable the naturalist 
at once to declare the soundings within certain limits, and 
without the aid of line or plummet.” 
In the British seas four distinct and well-marked zones of 
life succeed each other in vertical extension. The first of these 
is the littoral zone, equivalent to the tract between tide-marks, 
but quite as manifest in those portions of the coast-line where 
the tides have a fall of only a foot or two, or even less, as in 
districts where the fall is very great. This important belt, 
which again forms four subdivisions, and is inhabited by 
animals and plants capable of enduring periodical exposure to 
the air, to the glare of light, the heat of the sun, the pelting 
of rain, and often to being more or less flooded with fresh 
water when the tide has receded, claims many genera as well 
as species peculiar to itself. “The verge of continual air is gene- 
rally distinguished by the abundant presence of Fucus canali- 
culatus, among whose roots may be found crowds of small 
varieties of the periwinkle, called Littorina rudis, which indeed 
range out of the water considerably, and may be found adhering 
to rocks many feet above high-water mark.” The second sub- 
region is marked by the abundance of a small dark rigid sea- 
weed, called Lichina, paintiug the rock sides as if with a dingy 
stripe. With it we find the larger forms of Littorina rudis, 
abundance of the common limpet (Patella vulgata), the com- 
mon mussel (Mytilus edulis), and myriads of small seaside 
