When a medusa is inactive, its body gradually sinks to the bottom, being usually slightly heavier than the water in which it lives; but it is enabled to keep afloat by those rhythmic contractions of the bell with which we are so familiar. It seems that the medusæ are very sensitive to various external conditions, for they frequently disappear simultaneously from the surface water, and as suddenly reappear in shoals when the conditions are more favourable; but it is difficult to understand the causes which give rise to these remarkable movements. The medusæ are often termed the _Acalephæ_--a word which signifies ‘nettles,’ and they are popularly known as sea nettles. They all possess stinging cells, which are distributed most thickly in the tentacles, and some of the larger species are undoubtedly able to produce an impression on the bodies of unwary bathers, while almost all have the power of paralysing the living prey on which they feed. By far the commonest of the jelly-fishes of our seas is the beautiful blue medusa--_Aurelia aurita_. This species appears in enormous shoals during the summer, and large numbers are washed upon flat, sandy beaches. They vary in size from two or three inches to nearly a foot in diameter, and may be recognised from our illustration. The ‘bell’ is umbrella-shaped, and is so transparent that the stomach with its radiating canals may be seen through its substance. Around the margin there are little pigment spots which are supposed to be rudimentary eyes, and little cavities, containing a clear fluid, that are thought to serve the purpose of ears. [Illustration: FIG. 85.--_Aurelia aurita_] On the under surface may be seen the square mouth, furnished with four long and graceful frilled lips, which are richly supplied with stinging cells; also the four ovaries or egg-producing organs, rendered conspicuous by their violet colouring. The life history of _Aurelia_ is most interesting. The eggs are produced in pouches that communicate directly with the stomach-cavity, and these give rise to little ciliated larvæ that are ejected through the mouth, and then swim about freely in the water for a time. After this they settle at the bottom, lose their cilia, and become little cylindrical jelly-fishes, fixed by a short stalk-like foot to rocks or weeds. Numerous tentacles develop as the creatures increase in size, and a number of transverse furrows appear at the surface. The furrows gradually increase in depth until, at last, the body is broken up into several star-like discs, each of which floats away and develops into a new medusa. [Illustration: FIG. 86.--THE EARLY STAGES OF _Aurelia_] Other jelly-fishes, some of which are considerably larger than _Aurelia_, frequent our seas, and are often to be seen stranded on the beach. Two of these--_Rhizostoma_ and _Chrysaora_--are figured. Although they differ considerably in form from the blue aurelia, they closely resemble it in general structure and habits.