no HYDRA AND OBELIA. PHYLUM COELENTERATA 



which separates is simple in structure, but has the power of 

 repeating the structure of the parent body after fission. 



INDIVIDUALITY 



The term individual, whose application was in question in 

 the foregoing paragraph, 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. 

 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, its structure is essentially that of a 

 polyp, we might assume that each polyp is also an individual. 

 On the other hand, the whole polyp stock is a unit, and we 

 might consider it to be one individual, of which the separate 

 polyps are members, still regarding the medusa as an individual. 

 Of these alternatives the first is open to the objection that it 

 ignores the fact that the stock with all its polyps may be regarded 

 as a whole since it has common nourishment and reproductive 

 organs (the gonangia), and obliges us to regard as individuals the 

 blastostyles, which are morphologically equivalent only to parts 

 of other individuals. The second alternative is that which we 

 have adopted above. It may be stated as follows : ' Every con- 

 tinuous 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. 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 contains a number of 

 units meristically repeated, and the medusa is an individual 

 consisting of one unit of the same type as those which exist in 

 the polyp stock. All the products which one individual produces 

 by fission, such as the medusae of a single hydroid colony or the 

 whole family produced by the repeated fission of a Paramecium, 

 are collectively known as a clone. 



CCELENTERATA 



The phylum Coelenterata, to which Hydra and Obelia belong, 

 differs from all other groups in that its members have a body-wall 

 composed of two layers of cells only, the endoderm and ectoderm ; 



