The family of the Diphyidæ are colourless, and of such transparency that they are all but invisible when in the water, and are gelatinous masses clear as crystal when taken out of it. They are chiefly inhabitants of the warmer parts of the Pacific and Atlantic Oceans, but many fine specimens are found in the Mediterranean. Of these the Praya diphys is one of the most extraordinary (fig. 117). It has two large swimming-bells, their mouths turned backwards, with which the whole community is connected. They are nearly equal in size, soft, gelatinous, transparent, and colourless, rounded in front, open and truncated behind. The adjacent sides are parallel, with a groove between them, into which one end of the long tubular filiform body of the animal is fixed by slender tubes, through which a nourishing liquid passes into radiating canals in the bells, and from them into a circular canal at their margins, which are surrounded by a muscular contractile iris, like that in our eyes, which shuts and opens the bells. By the alternate absorption and ejection of the water the bells go head foremost, and regulate the motions of the whole compound animal. When both bells are active it goes straight forward; when the right hand bell is alone in action, it goes to the left, and _vice versâ_; in fact, the bells act as a rudder. The slender cylindrical body or axis of the Praya is so transparent, that the cavity and muscular fibres of its walls are distinctly seen. These animals are extremely contractile. Professor Vogt mentions an individual he met with at Nice more than three feet long, when extended on the surface of the water, which could contract itself into little more than a finger length. It was said to have had a hundred isolated groups of polypites with their appendages attached to it; but in general the Prayæ are not so long, and seldom have more than thirty or forty of these isolated groups, which are attached to the under-side of the long flexible body, and hang down like a rich and beautiful fringe. In the figure, the position of the numerous groups of polypites and their appendages are merely indicated by round marks and lines. In the body of the Praya diphys (fig. 117), as in that of the whole family, there is a nutritious liquid, which, by means of cilia, flows on its interior surface in two directions: it enters the canals in the two large bells, and supplies them with nourishment.