ALTERNATION OF GENERATIONS. ICj3 



VIII. Occurrence of these Alternations in Animals.— From sponges to 

 tiinicates such alternations occur. Beyond the latter, unless we wish to be very 

 subtle, they cease. It is necessary to be clear about the fact that asexual and 

 sexual reproduction may occur together in the same form. The common hydra 

 gives off buds in an entirely asexual way, but it is also a sexual animal, with 

 male and female organs. They may be periods of vegetative growth and 

 climacterics of sexuality in the same organism, without any alternation of 

 generations. 



It is possible that the term alternation of generations may be applied to some 

 of the phenomena observed in the Protozoa. Thus Brandt maintains that all 

 the colonial radiolarians, known as Sphcrrozoa, form on the one hand isospoi'es, 

 which are all equal and apparently parthenogenetic, and on the other hand 

 anisospores, which are large and small, — in fact, sexually dimorphic. He 

 believes — though the fact can not be called demonstrated — that two unequal 

 anisospores unite to form a double cell, a fertilized unit, which will produce 

 isospores again, and these the normal colony. The generation of these 

 sphaerozoa is further complicated (a) by division of the colonies, (d) by division 

 of .the individuals of young vegetative colonies, and (V) by the formation of 

 special " extra-capsular " reproductive bodies in young colonies. 



The history of the common fresh-water sponge (Spongilla), as told by 

 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 die "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 fertilize the ova of the 

 females. The fertilized ovum develops into a ciliated embryo, and this into an 

 asexual sponge, which produces the gemmules. 



Fig. 74. — The starting-point a fertilized 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 jellyfish alternations, 

 which we have already noticed, there are many complications of degree among 

 ccelenterates. 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 Fungia 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 reproductive 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. 



