HYDROZOA). This polyp can produce rootlets (stolons), from which new polyps are budded, and can also give rise to new polyps in other ways. The scyphistoma is a perennial organism, and at a given time of year may undergo a remarkable change, which varies according to whether the sup ply of food has recently been scarce or plentiful. In the f or mer case it differentiates from its upper end, a disc-like sec- . tion of its tissues which in time becomes free and swims away. If the food has been plentiful, however, a whole succession of such sections will be formed, one above the other like a pile of saucers, so that most of the substance of the polyp be comes converted into such. The segments separate from the parent (which in its dividing condition is known as strobila) suc cessively when sufficiently developed. Each of them is found on examination to constitute a small flattened medusa with eight long arms, and is termed an ephyra. It is quite unlike the adult Aurelia in shape even now, but assumes the fully developed con tozoa. The method by which the medusae are formed from the polyp, however, is a specialty of the Scyphozoa, and is quite unlike that adopted by the Hydrozoa.
The Scyphozoa constitute a large group of medusae of ex tremely varied and sometimes very elaborate structure. They possess in common, however, a number of features which dis tinguish them from the hydrozoan medusae, such as the absence of a velum (see HYDROZOA), and the presence, inside the coelen teron, of peculiar tentacles clothed by endoderm. The Scyphozoa
are cruciform in their symmetry, that is to say, all their organs are symmetrically arranged with relation to four main radii placed at right angles to one another. Their sex-organs are endodermal. They possess well-developed sense-organs, these including not only hollow tentaculocysts of a distinctive nature (see also COELEN TERATA and HYDROZOA), which occur in definite positions round the margin of the bell, but also ocelli or eye-spots, which attain, in the case of Charybdaea, an astonishingly high grade of develop ment, possessing cornea, lens, retina, and vitreous mass, and re calling in outline the structure of a vertebrate eye (fig. 3). In certain jellyfish (Rhizostomae) a curious condition of the mouth has arisen. By basal fusion of the four long arms which depend in so many of these jellyfish from the corners of the mouth (fig.
I), the mouth itself is obliterated, and food is taken in through a multitude of pores in the sur faces of the arms, which open into canals leading to the stomach.
Finally, in certain Scyphozoa (e.g., Haliclystus, fig. 4) the animal is not a swimmer, but has a stalk by means of which it attaches itself to weed and other objects; and here the anatomy is distinctive and is rather intermediate between that of a polyp and that of a medusa.
(T. A. S.)