INVERTEBRATA. 397 



from the egg. We see, therefore, that generation does 

 not strictly alternate with one gemmation, but with a 

 whole series of gemmations. 



12. Individuality. The study of the gemmarium of 

 Obelia gives rise to certain other important theoretical 

 considerations. We are accustomed from experience of 

 the higher animals to think of individual animals as 

 separate from one another, and incapable of continuing 

 to live when divided. In Obelia, however, we see a con- 

 siderable number of distinct animal units each with its 

 own organs, but all organically connected. We have seen 

 that the medusa is an individual constructed on the same 

 general plan as the sessile zooid, but it is separate and 

 independent. We can scarcely, therefore, deny the claim 

 of the zooids to be considered as individuals. At the same 

 time, these individuals are not so independent and com- 

 plete as those which are separate : the ordinary zooids 

 have no reproductive organs, the blastostyles have no feed- 

 ing organs, but are specialised for the production of medusae. 

 There is thus an individuality, an organisation of the 

 gemmarium, and the zooids have to a certain extent lost 

 their independence and completeness and become subor- 

 dinated to the needs of the aggregate to which they 

 belong. In other compound coelenterates this loss of 

 individuality has gone much further, so that the zooids 

 have become mere organs of the gemmarium, entirely 

 dependent for their existence on the life of the compound 

 of which they are members. It is thus difficult to give a 

 generally applicable definition of the individual, or organic 

 unit. It is rather a question of degree, the degree of cen- 

 tralisation. When the units are separate from one another 

 there is no difficulty, but when they are organically con- 

 nected the individuals may be reduced and subordinated 

 to a new organisation or individuality, that of the com- 

 pound. And in other cases, as in metamerism, it is 

 sometimes difficult to decide whether the similar parts 

 are to be regarded as individuals which are losing 

 individuality, or parts of a single individual which are 

 acquiring it. 



