HYDROMEDUSAE 



97 



luit a small jelly-fish or medusa (Figs. 37 and 41, D). This swims about, 

 Uvds and grows, and when fully developed has the following structure. 



The larger part of the body is concavo-convex, somewhat umbrella- 

 like, in form and is technically known as the umbrella. In the centre 

 of the concave surface, in place of the umbrella handle, is a short thick 

 projection the manubrium (Figs. 37 and 41, D, m). From the edge of 

 the umbrella extends a fringe of fine threadlike tentacles. Study of 

 the minute structure of the medusa shows it to be composed of the same 

 layers as the hydroid, only the mesogloea is greatly thickened forming 

 in any ordinary jelly-fish the mass of clear jelly from which the creature 

 gets its popular name. 



The ectoderm covers the whole external surface and it shows a distinct 

 advance in evolution from that of Hydra. More especially on the lower 

 concave surface of the umbrella the 

 myo-epithelial cells have their con- 

 tractile tails much more strongly 

 developed so as to form powerful 

 muscles, arranged concentrically 

 with the edge of the umbrella. By 

 means of these the medusa makes 

 the familiar pulsations, opening 

 and shutting, by which it swims 

 through the water. Further we 

 find distinct rudiments of a nervous 

 system. Here and there scattered 

 through the ectoderm are sensory 



cells (Fig. 38, s), tall and slender in shape, bearing at their outer 

 end a fine sensory hair of protoplasm which projects freely into the 

 surrounding water and serves to receive impressions from the outer 

 world. The sensory cells are prolonged at their inner end into a proto- 

 plasmic thread a nerve-fibre (a.f) which joins with many others to form 

 a plexus or network lying near the inner limit of the ectoderm all over the 

 creature's body. Here and there such a fibre may be traced to a cell lying 

 in the deep layers of the ectoderm (Fig. 38, g.c), or beneath it entirely a 

 ganglion-cell, probably to be interpreted as a sensory cell which has 

 become withdrawn from the surface. Sometimes it is possible to trace 

 another nerve fibre (Fig. 38, e.f) passing away from the ganglion cell 

 and leading to a muscle cell (m). Here we have an excellent example 

 of a nervous mechanism of the simplest possible type, consisting of a 

 nerve centre in this case a single ganglion cell and in relation with it 

 a sensory or afferent path along which come messages or impulses from 



m 



FIG. 38. 



Illustrating the nervous mechanism of 

 Medusae, a.f, Afferent (sensory) nerve fibre ; 

 e.f, efferent (motor) nerve fibre ; g.c, ganglion 

 cell ; m, muscle cell ; s, sensory cell. 



