238 HYDROID POLYPES LESS. 



only of soft ectodermal and endodermal cells, be so weak as 

 to be hardly able to bear its own weight even in water. To 

 remedy this a layer of transparent, yellowish substance of 

 horny consistency, called the cuticle, is developed outside 

 the ectoderm of the stem, extending on to the branches and 

 only stopping at the bases of the hydranths and medusae. 

 It is this layer which, when the organism dies and decays, 

 is left as a semi-transparent branched structure resembling 

 the living colony in all but the absence of hydranths and 

 medusae. The cuticle is therefore a supporting organ or 

 skeleton, not, like our own bones, formed in the interior 

 of the body (endo skeleton}, but like the shell of a crab 

 or lobster lying altogether outside the soft parts (exo- 

 skeleton). 



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

 many organisms, such as Amceba and Hsematococcus, 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 Amceba 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 side's and with the mesoglcea 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. 



