IV MEDUSA 307 



Bougainvillea, on the other hand, are in close contact with 

 their neighbours on all sides and with the mesogloea at their 

 inner ends, so that it is not surprising to find the secretion 

 of skeletal substance taking place only at their outer ends. 

 As the process takes place simultaneously in adjacent cells, 

 the result is a continuous layer common to the whole 

 ectoderm instead of a capsule to each individual cell. It is 

 to an exoskeletal structure formed in this way, i.e. by the 

 secretion of successive layers from the free faces of adjacent 

 cells, that the name cuticle is in strictness applied in multi- 

 cellular organisms. 



The medusee (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. 77, d, msgl) covered on both its inner 

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

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

 organ or manubrium (Fig. 76, c and Fig. 77 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. 77, ent. cav) open- 

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

 the mouth {?nth), 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, 76, c, rad. c, and Fig. 77, d and d' rad) which extend 



X 2 



