C(ELENTERATA. 151 



A Iternations of generations. 



Alternation of generations is of common occurrence amongst the 

 Hydrozoa, and something analogous to it has been found to take 

 place in Fungia amongst the Actinozoa. It is not known to occur in 

 the Ctenophora. 



The chief interest of its occurrence amongst the HydromedusaB 

 and Siphonophora is the fact that its origin can be traced to a 

 division of labour in the colonial systems of zooids so characteristic 

 of these types. 



In the Hydromedusse an interesting series of relations between 

 alternation of generations and the division of the zooids into 

 gonophores and trophosomes can be made out. In Hydra the 

 generative and nutritive functions are united in the same individual. 

 The generative swellings in these forms cannot, as has been ably 

 argued by Kleinenberg, be regarded as rudimentary gonophores, 

 but are to be compared to the generative bands developed in the 

 Medusae around parts of the gastro-vascular system. A condition 

 like that of Hydra, in which the ovum directly gives rise to a form 

 like its parent, is no doubt the primitive one, though it is not so 

 certain that Hydra itself is a primitive form. The relation of Hydra 

 to the TubularidsB and Campanularidse may best be conceived by 

 supposing that in Hydra most ordinary buds did not become detached, 

 so that a compound Hydra became formed ; but that at certain 

 periods particular buds retained their primitive capacity of becoming 

 detached and subsequently developed generative organs, while the 

 ordinary buds lost their generative function. 



It would obviously be advantageous for the species that the 

 detached buds with generative organs should be locomotive, so as to 

 distribute the species as widely as possible, and such buds in connection 

 with their free existence would naturally acquire a higher organi- 

 zation than the attached trophosomes. It is easy to see how, by a 

 series of steps such as I have sketched out, a division of labour might 

 take place, and it is obvious that the embryos produced by the 

 highly organized gonophores would give rise to a fixed form from 

 which the fixed colony would be budded. Thus an alternation 

 of generations would be established as a necessary sequel to such 

 a division of labour. To test the above explanation it is necessary 

 to review the main facts with reference to alternations of generations 

 amongst the HydromedusaB. 



Hydromedusae 1 - In many instances amongst the Tubularidae, 

 Sertularidse and Campanularidae medusiform buds are produced 

 which become detached and develop sexual organs. 



Such Medusse are divided into two great groups, the Ocellata and 

 Vesiculata, according to the characters of the marginal sense organs. 



1 For a full account of this subject the reader is referred to the beautiful memoir 

 of Allman (No. 149). 



