ALTERNATION OF GENERATIONS. 209 



Marshall, is one of many vicissitudes. In autumn the sponge begins to 

 suffer from the cold and scarcity of food. It dies away ; but some of the 

 units save themselves, and, in a sense, the parent, by forming the 

 " gemmules" we have already noticed. These winter in a quiescent state 

 within the parental corpse, but in spring they get out of the debris, and 

 start male or female sponges. The males are short-lived, but their male 

 elements fertilise the ova of the females. The fertilised ovum develops 

 into a ciliated embryo, and this into an asexual sponge, which produces the 

 gemmules. 



The starting-point a fertilised ovum, which develops into 

 A = asexual sponge, which forms only 

 G— gemmules, which develop into 

 S -^ male and female sponges. 



Besides the hydroid and medusoid, the hydra-tuba and jelly-fish alterna- 

 tions, which we have already noticed, there are many complications of 

 degree among C(ielenterates. The medusoid stage degenerates by subtle 

 gradations, ceasing to be free, and eventually becoming what, if its 

 history were not known, would be called an organ rather than a " person " 

 of the colony. Furthermore, it may itself take to budding, and continue 

 the asexual habit of the hydroid from which it springs. Outside the 

 Hydrozoa, genuine alternation of generations does not occur, unless that 

 described by Semper for P'ungia corals be accepted as such. 



A very interesting alternation has been recently described by W. K. 

 Brooks in a remarkable medusa {Epenthesis macradyi). On the reproduc- 

 tive organs of this swim-bell there grow, like parasites, what are exactly 

 comparable to the reproductive buds (blastostyles) of a hydroid, and these 

 form medusoids by budding. The result is a compound colony, which 

 approaches the Siphonophora. The process recalls and surpasses the 

 apogamy of a few ferns. 



Among worm- types, the strict alternation of generations in some of the 

 marine chstopods (syllids), the more complicated phenomena of so many 

 trematodes, the sexual rhythms of that peculiar threadworm Aiigiostomtim^ 

 have been already discussed. It is necessary, however, to state the case for 

 tapeworms, which are usually included among the examples of alternation 

 of generations. The usual view is, that the embryo of a tapeworm develops 

 into an asexual bladder- worm, which asexually buds off a " head," or more 

 than one. Such a " head," passing to another host, buds off asexually 

 the chain of reproductive joints or sexual individuals which constitute a 

 tapeworm. Asexual bladder-worm, asexual " head," and sexual joints, 

 form the series. That there is a genuine alternation of generation is 

 believed by some authorities, but there are emphatic difficulties against 

 this supposition, except in the occasional occurrence of a bladder-worm 

 with several "heads," each of which may develop into a tapeworm. The 

 case is well stated by Hatchett Jackson in his monumental edition of 

 Rolleston's *' Forms of Animal Life," and we accept his verdict that there 

 is really one individual throughout, except when asexual multiplication of 



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