THE CCELENTERATA 169 



branch and anastomose in a network, the hydrorhiza, which in its turn 

 serves for the origin of a number of new hydrocauli. In this way 

 there is produced the complex colony, or stock, with which we started. 



Before leaving Obelia there is one further phenomenon 

 concerning it that calls for notice. Starting with the hydrula we 

 find that by a process of budding, similar to that which in Hydra 

 produces a number of separate individuals, there is formed a large 

 colony of hydranths joined by the coenosarcal canals. It is a matter 

 of opinion whether we regard the whole stock as one individual 

 with the hydranths, blastostyles and medusae, as separate parts or 

 organs, or look on each of them as an individual in itself, in organic 

 unity with its fellows. The majority of zoologists adopt the latter 

 point of view, and regard the colony as the result of the asexual 

 reproduction of the one person. Further asexual multiplication 

 leads to the formation of the blastostyles, and yet again to the 

 medusae. The whole colony, together coming from the one hydrula 

 and producing medusae, forms the asexual generation or agamobium. 

 The medusae, however, do not reproduce by budding, but by the 

 formation of sexually differentiated gametes, and so they constitute 

 the sexual generation or gamobium. Thus the life history of Obelia 

 includes an asexual generation alternating with a sexual one ; it is 

 an instance of metagenesis or the alternation of generations. It 

 should be noted in passing, however, that such an alternation is not 

 identical with that with which the botanist has to deal- in certain 

 plants, e.g. the fern. In these plants both the generations produce 

 reproductive cells. The form known as the sporophyte produces 

 single cells, the spores, which are capable of giving rise to an entirely 

 new and different individual without first undergoing any kind of 

 fertilisation. This second form, the gametophyte, cannot produce 

 spores, but only sexual gametes, which must unite to form a zygote 

 before development can proceed any further. Thus we have an 

 alternation of generations often very different in appearance, each 

 capable of giving rise to reproductive cells, a condition which is not 

 paralleled in the animal kingdom. Moreover, among animals, one 

 generation or the other, only rarely both, is capable of vegetative 

 reproduction by budding in a very similar way to Hydra. 



We have now completed our review of the diploblastic 

 Ccelenterate types Hydra and Obelia, and have noted in them not 

 only a considerable advance over the Protozoa, but also a certain 

 progressive series of specialisations within the phylum itself, leading 

 to the production of definite localised organs for the performance of 

 certain functions. Thus in these lowly members of the animal 

 kingdom we see indications of the beginning of that complexity of 

 organisation brought about by the physiological division of labour 

 that becomes more and more marked in the higher groups. 



