HYDRA AND OBELIA. POLYPS AND MEDUSM 183 



asexual reproduction of a protozoon, or an insect such as 

 the green fly {Aphis) whose ova develop for generations 

 without fertilisation, are parts of one individual, but also 

 that the "identical" twins formed by the dividing of a 

 single ovum are not, even in the case of man, separate 

 individuals, and this amounts to a reductio ad absurdum. 

 (b) In taking the view that each polyp of the Obelia stock 

 is an individual — which view may be stated to the effect 

 that " an individual is a structure which has all the organs 

 of life save sometimes those of reproduction, or is a modi- 

 fication of such a structure " — we are met with two diffi- 

 culties : (i) It ignores the fact that the stock equally with 

 each of its polyps may be regarded as a whole, since it has 

 common nourishment and reproductive organs (gonangia). 

 (ii) It obliges us to regard as individuals structures like 

 the blastostyles which are morphologically only equivalent 

 to parts of other individuals, whereas if we regard the 

 whole stock as an individual the blastostyles can easily be 

 regarded as organs of that individual, (c) The remaining 

 alternative (the second of those stated in the preceding 

 paragraph) may be stated as follows : " Every continuous 

 mass of living matter which arises normally by fission is 

 an individual." That view has the advantage that it does 

 not force us to create artificial units of any kind, and it is 

 the one which we shall adopt. According to it, the act 

 which makes an individual is the act of fission by which 

 it becomes independent of its parent, and fertilisation is 

 the blending of two undeveloped individuals into one, 

 while the polyp stock is an individual which consists of a 

 number of units meristically repeated, and the medusa 

 is an individual consisting of one unit of the same type 

 as those which compose the polyp stock. 



Merism is shown in the formation of new zooids both 

 in Hydra and in Obelia. In Hydra all the zooids separate 

 and all are alike. In Obelia only some of the zooids 

 separate, and these — the medusa — repeat the original 

 structure of the body with great modification. In Hydra 

 the life-cycle by which an individual reproduces another 

 in its own likeness involves a single act of fission. In 

 Obelia it involves two. Reproduction and merism are 

 results of the same process, and it is, after all, a matter of 



