ALTERNATION OF GENERATIONS. 41 



of Mr. Lubbock* are against the supposition that the ephippia of Daplmia are gemmae. He 

 has traced the formation of the ephippial ova in the ovary as true ova ; but admits at the 

 same time that they are possibly agamic. The external resemblance of these ephippia with 

 the statoblasts of the Polyzoa is singularly striking; but, witli the very different origin of the 

 two sets of bodies, we must take care not to attribute a greater share of significance to this 

 resemblance than it really desei'ves.t 



The reproductive phenomena of the fresh-water Polyzoa may be thus classed under three 

 distinct heads : 



Sexual reproduction . . By true ova. 



By gemmae which at once proceed to the full 



term of their destined development. 

 By statoblasts or gemmse in which the 



developmental activity remains for a 



period latent. 



Non-sexual reproduction 



In Cristaiella and Lojihojnis I have frequently witnessed the multiplication of a colony by 

 a process of self-division. In CrisfateUa this commences by a constriction which takes place 

 generally towards the middle of the colon)', and which gradually deepens till at last it divides 

 the entire mass into two separate portions, which move off in opposite directions. In 

 Lophopus the process is very similar ; large specimens of this polyzoon have the endocyst 

 constricted at intervals so as to give to the colony the appearance of a variously lobed body 

 enveloped by the gelatinoid ectocyst. It is at the point of these constrictions that the self- 

 division takes place, separating the entire colony into two or more smaller ones. 



It may, perhaps, be thought that I ought to have enumerated this multiplication of 

 colonies by a process of self-division as a fourth form of reproduction ; a little consideration, 

 however, will show that this is nothing more than a reproduction by buds, with the separation 

 of the buds in masses. It is analogous to the gemmiparous reproduction of Hydra, and must 

 not be confounded with the true fissiparous reproduction of the lower forms of simple animals. 

 In the Polyzoa the colony thus extends itself by the production of gemmse, which after 

 development remain permanently adherent; it establishes new colonies by ova and statoblasts, 

 and by ordinary gemmfe which ultimately become detached. 



If we attempt to correlate the individual phenomena now described in connection with 

 the reproduction of the Polyzoa, we cannot but be struck with some remarkable analogies which 

 would seem to bring the whole process of generation and gemmation in these animals within 

 the domain of the so-called " Law of alternation of generations." We have, first, as the 

 immediate result of the development of the ovum, a ciliated sac-like embryo, resembling in 

 form and habit an infusorial animalcule : it is a non-sexual zooid. From this is produced 

 subsequently, by a process of gemmation, another form of zooid, namely, the polypide, with a 



* An account of tlie two metliods of reproduction in Daphnia, and of tlie structure of tlie 

 " epliippium." By Jolin Lubbock, Esq. Abstract in ' Proc. Roy. Soc.,' Jan. 29, 1857, vol. viii. 



t In the mode of development of the statoblasts from the funiculus of the polypide, we are 

 involuntarily reminded of the development of the chains of salpa-buds from the stolon of the solitary 

 salpffi. 



6 



