698 COELENTERATA—II. CNIDARIA. 



With their tentacles, no doubt amply provided with stinging threads, 

 they catch the food which, being themselves unable to eat, they pass 

 on to the mouth of the central polyp. Within the body of this central 

 polyp the food is digested, but not for itself alone, as the products of 

 digestion, flowing along the many canals of the colony, feed all its 

 members. There are no free swimming MeduEse connected with these 

 Hydrocorallia. 



We return now to the reproductive individuals, which, in many Hydroid 



colonies, carry and distribute the eggs (Fig. 6, B). These, when seen 



swimming about independently, are so like the larger Jelly- 



Hydromedusae. fish belonging to the Scyphozoa that they were long mistaken 



for them. The general plan of the body, indeed, is very 



similar and is worth describing, especially in order to show what changes are 



necessary in the ordinary Hydra-like body to produce a swimming bell. 



In all Medusfe the general shape of the body is due to the great thickening 

 of the middle supporting layer in the body wall, which we described in con- 

 nection with the Hydra. Instead of a long body like that of the Hydra, 

 running like a tube from the mouth to the bottom of the gastric sac, we have 

 one flattened out like an umbrella, the upper part of the umbrella corre- 

 sponding with the former closed end of the tube, while the mouth is at the 

 tip of a short stalk or handle to the umbrella. The transformation of the 

 Hydra into the Medusoid can be imagined by supposing the mouth pro- 

 minence between the tentacles to be pushed down into the body, which 

 expands laterally, till the whole forms a bowl with the oral prominence 

 rising up in its base; the tentacles take up positions on the rim of the bowl. 

 The stomach lies partly in the mouth stalk or manubrium (handle) and partly 

 in the body of the umbrella. Here it sometimes forms a cavity, and a number 

 of canals run from it radially out, like spokes, to the edge of the umbrella, 

 there to meet a canal which runs round the margin. When food is taken 

 into the stomach the nutritive fluid derived from it passes through the radial 

 canals to the circumferential canal and nourishes the whole body. The 

 number of these radial canals in the Hydromedusse is usually small (two, 

 fouc, or eight), and these are little if at all branched. The margin of the 

 body is fringed with tentacles, and carries the so-called "marginal bodies," 

 some of which are minute bags (lithocysts) containing bodies called otoliths, 

 whose function is usually said to be auditory, but is more probably that of 

 regulating the position of the animal in the water. Other brilliantly coloured 

 bodies, af.'ain, have been named "eye-spots." The Hydromedusse were 

 formerly distinguished from the Jelly-fish proper as "naked-eyed" Medusse, 

 because in them these "eyes" are exposed, whereas in the other Jelly-fish, 

 or "covered-eyed" Medusas, they are protected by a hood-like lappet of 

 gelatinous tissue. Simple tactile organs are also found along the margin of 

 the Hydromeduste, round which a double nerve ring runs. 



Another characteristic of the Hydromedusse is the velum (veil), a thin, 

 muscular membrane which hangs down from the margin of the umbrella, 

 slanting inward toward the manubrium. 



The swimming movements of these animals also deserve notice. The 

 lower concave surface of the "umbrella" is supplied with a very thin layer of 

 contractile tissue or muscle, by the contraction of which the umbrella is 

 partially closed. This action, performed suddenly, drives the water out from 

 under the umbrella and propels the Medusa along. 



The eggs, in the Hydromedusaj, form along the radial canals, or on the 



