ANATOMY OF THE POLYP I05 



THE MEDUSA 



The generative organs are not borne by the polyps, but by 

 special bodies, which originate as members of the colony, are 

 set free by breaking away as the buds of Hydra are, and carry 

 out sexual reproduction as independent individuals. These differ 

 widely from the polyps, and are, indeed, so unlike them that their 

 origin from the colony would never have been guessed unless it 

 had been seen to take place, but they are fundamentally similar 

 in structure (see p. 112). They are small jelly-fish or medusae (Figs. 

 70, 71). Each has the shape of a mushroom with a short, thick 

 stalk and a fringe of tentacles around the edge. The convex upper 

 side is called the exumbrella, the concave lower side the sub- 

 umbrella, the whole disc-shaped upper part the umbrella, and the 

 stalk the manubrium. Around the edge of the subumbrella a low 



Fig. 70. — A medusa of Obelia, magnified. 



ridge projects inwards. This is the velum and represents a much 

 larger structure in the same region of many other medusae. At the 

 end of the manubrium is the mouth, which leads by a tubular 

 gullet along the manubrium to a stomach in the middle of the 

 body. From this four radial canals run outwards to a ring canal at 

 the edge of the umbrella. The lining of all these internal spaces 

 consists of endoderm, and the radial canals lie in a sheet of 

 endoderm, known as the endoderm lamella. In fact we may regard 

 the internal cavities of a medusa as corresponding to the enteron 

 of a polyp in which the walls have come together over a large area, 

 leaving certain spaces which form the gullet, stomach, and canals. 

 The whole outside of the body and tentacles is covered with ecto- 

 derm. Between the ectoderm and the endoderm is a layer of jelly, 

 which is very thick, especially on the exumbrella side. The medusa 

 may be compared to a polyp which is greatly widened and short- 

 ened, the walls of the wide, fiat enteron coming together in places, 

 as we have seen, and the structureless lamella increasing in 



