The composition of the waters has also a most important effect on the distribution of aquatic animals, as 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 assemblage 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 generally distinguished by the abundant presence of Fucus canaliculatus, 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, painting 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 common mussel (Mytilus edulis), and myriads of small seaside barnacles, often striping the sea-wall in a broad white band. "Where the shore shelves a little, and rocky ledges decline gradually into the sea, the common mussel delights to live, firmly anchored by its byssal cable in the crevices of rocks or among masses of gravel, the pebbles of which are tied together by its silky filaments." The rock sides and the floors of transparent pools are here often thickly coated with a nullipore, forming a hard pale red crust. The region of half-tide forms a third subdivision of the littoral zone, and is exceedingly prolific in marine animals and plants. "Here we find Fucus articulatus, with its graceful even-edged rich brown fronds, mingled occasionally with the less elegant Fucus nodosus. Here limpets throng, and dog-periwinkles (Purpura lapillus) crawl observantly, seeking to bore more passive mollusks and extract their juicy substance. This is the home of the best of periwinkles, the large black Littorina littorea, gathered in thousands for the London market. On our western coasts we find it in company with the purple-striped top-shell (Trochus umbilicatus), and towards the south with the larger Trochus crassus. Here also sea-anemones love to expand their many-armed disks, often glowing with the most brilliant colours." A fourth sub-region succeeds, the lowest belt above low-water mark, and is distinguished by the presence of the black saw-toothed sea-weed (Fucus serratus), so much used in the packing of lobsters for market. On its fronds creeps the lowest in grade of the periwinkles, the variously tinted Littorina neritoides, exhibiting every colour in its obtuse and thickened shell.
Limpet.
Periwinkle.
"At the verge of low-water mark, immediately below it, wherever the coast is rocky, there are all round the British shores, within a space of a few inches, a remarkable series of more or less distinctly defined belts, each consisting of a different species of sea-weed. These in succession are, the Laurencia pinnatifida uppermost; then the green Conferva rupestris; then the elegant and firm, often iridescent, fronds of Chondrus crispus; and, lowermost, the thong-weed or Himanthalia lorea."
Succeeding the shore-band, or littoral zone, we have the region of the great laminaria or tangle forests, or in sandy places the waving meadows of zostera, or grass-wrack. It extends from the edge of low water to a depth varying in different localities, but seldom exceeding fifteen fathoms, and is itself divided into sub-regions, marked by belts of differently tinted algæ. This zone above all others swarms with life, and is the chief residence of fishes, mollusks, crustaceans, and invertebrata of all classes, remarkable for brightness and variety of colouring. "Here," says Mr. Godwin Austen, "is the chosen haunt of the nudibranchiate mollusks, animals of exceedingly delicate texture, extraordinary shapes, elegance of organs, and vividness of painting. Their bodies exhibit hues of a brilliancy and intensity such as can match the most gorgeous setting of a painter's palette. Vermilion red, intense crimson, pale rose, golden yellow, luscious orange, rich purple, the deepest and the brightest blues, even vivid greens and densest blacks, are common tints, separate or combined, disposed in infinite varieties of elegant patterns, in this singular tribe. Our handsomest fishes are congregated here, the wrasses especially, some of which are truly gorgeous in their painting. Here are gobies and more curious blennies, swimming playfully among these submarine groves. Strange worms crawl serpent-like about their roots, and formidable crustacea are the wild beasts who prowl amid their intricacies. The old stalks, and the surfaces of the rocky or stony ground on which they usually grow, are incrusted like the trunks of ancient trees or faces of barren rocks with lichenous investments. But whereas in the air these living crusts are chiefly if not all of vegetable origin, in the sea they are more often constructed out of animal organisms. Some of them are sponges, others are true zoophytes, others polyzoa or bryozoa, beings that have proved to belong to the class of mollusks, however unlike they may seem to shell-fish.
"In the middle and lower part of the Laminarian region around our shores the tangles become less plentiful as we descend, and at last become exceptional and disappear. But other sea-weeds are very abundant, especially those that delight in red or purple hues. Tender sea-mosses, exquisitely delicate in form and colouring, abound. Where none of these are very plentiful, we often find the coral-weed or nullipore in vast quantities, and assuming many strange modifications of form. Among these vegetable corals numbers of shells and articulate animals delight to live, and probably not a few feed upon their stony fronds. The Lima, a shell-fish related to the scallop, gathers the broken branches by means of prehensile tentacles, and constructs for itself a comfortable nest lined with a woven cloth of byssal threads. Numerous fishes resort to these rugged pastures in order to deposit their spawn among the gnarled branchlets."