36 THE ANATOMY OF INVERTEBRATED ANIMALS. 



It thus gives rise to a series of independent organisms (B, 

 B B, . . .), which are more or less different from A, and 

 which sooner or later acquire generative organs. From their 

 impregnated germs A is reproduced. The process thus de- 

 scribed is what has been termed the "alternation of genera- 

 tions" under its simplest form — for example, as it is exhibited 

 by the Salpce. In more complicated cases the independent 

 organisms which correspond with B may give rise agamo- 

 genetically to others (BJ, and these to others (BJ, and so 

 on (e. g., Aphis). Bat, however long the series, a hnal term 

 appears which develops sexual organs, and reproduces A. 

 The " alternation of generations " is, therefore, in strictness, 

 an alternation of asexual with sexual generation, in which 

 the products of the one process differ from those of the 

 other. 



The Hydrozou offer a complete series of gradations be- 

 tween those cases in which the term B is represented by a 

 free, self-nourishing organism (e. g., Cyancea), through those 

 in which it is free but unable to feed itself ( Calycophoridm), 

 to those in which the sexual elements are developed in bodies 

 which resemble free zooids, but are never detached, and are 

 mere generative organs of the body on which they are devel- 

 oped ( Cordylophora). 



In the last case the " individual " is the total product of 

 the development of the impregnated embryo, all the parts of 

 which remain in material continuity with one another. The 

 multiplication of mouths and stomachs in a Cordylophora no 

 more makes it an ao*2;reofation of different individuals than 

 the multiplication of segments and legs in a centipede con- 

 verts that Arthropod into a compound animal. The Cordy- 

 lophora is a differentiation of a whole into many parts, and 

 the use of any terminology which implies that it results from 

 the coalescence of many parts into a whole is to be depre- 

 cated. 



In Cordylophora the generative organs are incapable of 

 maintaining a separate existence ; but in nearly-allied Hydro- 

 zoa the unquestionable homologues of these organs become 

 free zooids, in many cases capable of feeding and growing, 

 and developing the sexual elements only after they have un- 

 dergone considerable changes of form. Morphologically, the 

 swarm of Medusa* thus set free from a Hydrozoon are as 

 much organs of the latter as the multitudinous pinnules of a 

 Comatula, with their genital glands, are organs of the Echi- 

 noderm. Morphologically, therefore, the equivalent of the 



