xxn STRUCTURE OF A MEDUSA 239 



The medusae (B, med, and c), mentioned above as occur- 

 ring on lateral branches of the colony, are found in various 

 stages of development, the younger ones having a nearly 

 globular shape, while when fully formed each resembles a 

 bell attached by its handle to one of the branches of the 

 colony and having a clapper in its interior. When quite 

 mature the medusae become detached and swim off as little 

 jelly-fishes (c). 



The structure of a medusa must now be described in 

 some detail. The bell or umbrella (c) is formed of a gela- 

 tinous substance (Fig. 55, D, insgl) covered on both its inner 

 surface or sub-umbrella and on its outer surface or ex-umbrella 

 by a thin layer of delicate cells (ect). The clapper-like 

 organ or manubrium (Fig. 53, c and Fig. 55 D and D', mnb] 

 is formed of two layers of cells, precisely resembling the 

 ectoderm and endoderm of Hydra, and separated by a thin 

 mesogloea; it is hollow, its cavity (Fig. 55, D, ent. cav) open- 

 ing below, i.e. at its distal or free end, by a rounded aperture, 

 the mouth (mt/i), used by the medusa for the ingestion of 

 food. At its upper (attached or proximal) end the cavity of 

 the manubrium is continued into four narrow, radial canals 

 (Fig. 53, c, rad. c, and Fig. 54, D and D' rad) which extend 

 through the gelatinous substance of the umbrella at equal 

 distances from one another, like four meridians, and finally 

 open into a circular canal (dr. c} which runs round the edge 

 of the umbrella. The whole system of canals is lined by a 

 layer of cells (Fig. 55, D and D', end) continuous with the 

 inner layer or endoderm of the manubrium ; and extending 

 from one canal to another in the gelatinous substance of the 

 umbrella is a delicate sheet of cells, the endoderm-lamella 

 (D', end. Id). 



From the edge of the umbrella four pairs of tentacles 

 (Fig. 53, c and Fig. 55, D, t) are given off, one pair corres- 



