HYDRA AND OBELIA. POLYPS AND MEDUSAE 181 



parts — the blastostyles and most of the ccenosarc — which 

 arise in the course of development to serve the entire body 

 and are wholly dependent upon it. The relation between 

 development and reproduction which this view involves 

 may be stated as follows. Any part of an organism, from 

 the smallest organ to the whole body, is liable to be 

 repeated, with or without differences between the repeated 

 parts. This phenomenon has been called merism : we 

 have seen it in cilia, trichocysts, contractile vacuoles, cells, 

 limbs, zooids, etc. Sometimes, as in cilia, cells, or zooids 

 of the same kind, it has not involved differences. Some- 

 times, as in cells or limbs or zooids of different kinds, it has 

 involved differences between the repeated parts. Some- 

 times the parts are present in their full number from the 

 first; sometimes they increase in number as growth goes 

 on. From time to time every organism produces a part 

 which not only repeats its whole structure, but also 

 separates from it by an act of fission. This process is 

 reproduction. In some cases of reproduction, as in the 

 budding of Hydra, the repetition of structure takes place 

 before separation. In other cases, as in the formation of 

 germs, 1 the part which separates is simple in structure, 

 but has the power of repeating the structure of the parent 

 body after fission. It will be seen that merism is one 

 of the results of the process which we have called develop- 

 ment. Working on the large scale development produces 

 from the reproductive body a body like that of the parent; 

 working on a smaller scale it produces from portions of the 

 body-substance structures which are like other structures 

 within the body. It is also at work in regeneration. 



We have seen that the term individual is often applied 

 utv t0 tne severa -l hydranths of Obelia. This term 

 has been used in zoology with very different 

 meanings, which are well illustrated by the life-history of 

 the hydroids. An individual is a single complete living 

 being. There are in Obelia three things that might claim 

 to be this : (i) Since the medusa carries out all the 

 functions of life in itself, it seems natural to assert that 

 it is a complete living being, and since, as we have seen, 



1 In this case there may be the additional complication that two such 

 separated parts so develop as to produce but one body after fusion, 



