The transparency and colourlessness of marine animals which float near the surface is, on the other hand, obviously useful, and to this group our jelly-fishes belong. Not only do they escape observation by their transparency and general absence of colour, but some actually have a blue transparent colouring which blends with the blue colour of the sea. Such are the gas-holding, bladder-like sac as large as your fist called the "Portuguese man-of-war," and the little sailing Velella, both of which float, and even protrude above the surface, so as to catch the wind. Others are only semi-transparent, and others are marked with strong red, brown, or yellow streaks. Many of the smallest kinds of jelly-fish have eyes which are bright red in colour.
[Illustration: FIG. 7.--A common British Jelly-fish.
Aurelia aurita, usually as large as a breakfast-plate and often larger.]
The animals to which the name "jelly-fishes" is now more or less strictly applied are (as that fine zoologist Aristotle knew) in their structure closely similar to the sea-anemones, but even simpler. They are called the Medusæ by naturalists. Their disk-like bodies are largely formed by a jelly-like material, on the surface of which are stretched delicate transparent skin, nerves, and delicate muscles, whilst in the middle of the disk, on the surface which faces downwards as the creature floats, is the mouth, leading into a relatively small pouched cavity excavated in the jelly, from which a delicate system of canals is given off, and radiates in the jelly of the disk. There is, as in the sea-anemones, only one continuous cavity. The edge of the disk is beset with fine, sensitive tentacles, sometimes many feet in length, and the lips of the mouth are often drawn out into a sort of depending trunk, or into four large tapering lobes or lips of jelly, which, with the longer tentacles, are used for seizing prey. The commonest jelly-fish on our coast--so common as to be "the" jelly-fish _par excellence_--is often to be seen left on the sands by the receding tide or slowly swimming in quiet, clear water at the mouth of a river in enormous numbers. It is known as "Aurelia" (Fig. 7). It is as big as a cheese-plate, and the four pouches connected with the stomach are coloured pink or purple, and appear in the middle of the circular plate of jelly, like a small Maltese cross. The reproductive particles (germ-cells and sperm-cells) are produced in that coloured region, and escape by the mouth. There is a fringe of fine, very short tentacles round the edge of the disk, and they, as well as the great lobes of the mouth, are provided with innumerable coiled-up stinging hairs or "thread-cells," similar to those of the sea-anemones, which led Aristotle to call both groups "sea-nettles." Eight stalked eyes are set at equal intervals around the disk.