240 HYDROID POLYPES LESS. 



the body (endoskehton\ but like the shell of a crab or lobster 

 lying altogether outside the soft parts (exoskeleton). 



As to the mode of formation of the cuticle : we saw that 

 many organisms, such as Amoeba and Haematococcus, form, 

 on entering into the resting condition, a cyst or cell-wall, by 

 secreting or separating from the surface of their protoplasm 

 a succession of layers either of cellulose or of a transparent 

 horn-like substance. But Amoeba and Haematococcus are 

 unicellular, and are therefore free to form this protective 

 layer at all parts of their surface. The ectoderm cells of 

 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 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 medusa must now be described in 

 some detail. The bell (c) is formed of a gelatinous sub- 

 stance (Fig. 55, D, msgl) covered on both its inner and 



