ALTERNATION OF GENERATIONS. 131 



The great function of the germinal or protomorphic zooids 

 is the evolution of the embryos of higher forms, of 

 which they serve as budding-stocks ; that of the sexual or 

 gamomorphic zooids is the development of ova and sperma- 

 tozoa. These ends accomplished, their vitality ceases, 

 while the typical organism, the offspring of the former class, 

 or the parent stock of the latter, as the case may be, has a 

 much more permanent duration, and may go on for a long 

 time in perfect vigour, sending off crop after crop of ova, 

 or of sexual gemmae, according to its mode of propagation. 



5. The distinctness of these varieties of " alternation' ' 

 is further shown by their occasional co-existence in the same 

 species. Such an association occurs, for instance, in some 

 Cestoid worms. This is apparent from the sketch before 

 given of the development, first of the Cysticercus, and then 

 of the Tsenia, from the six-hooked contractile vesicle, 

 discharged from the egg of the latter. The vesicle, as was 

 shown, is first transformed into the " cyst," which represents 

 the Kedia or protomorphic zooid of the Trematoda, and 

 then buds off the Taenia-head (the orthomorphic or typical 

 form). From the latter are again derived, by a second 

 gemmation, the cucurbitiform or sexual segments making 

 up the body of the Tapeworm, which represent the gamo- 

 morphic zooids of the Polypifera. 



This species was selected in illustration, as the best known, 

 and, in some respects, the most characteristic of the order. It 

 certainly exemplifies as well as any the development of the 

 gamomorphic structures, but the gemmation in the precur- 

 sory or protomorphic stage is a much less conspicuous fea- 

 ture in it than in many other species, from the circumstance 

 that there is ordinarily formed but one gemma, and that 

 this is never detached from the primary cyst, till the latter 

 disappears on the animal entering on its tsenioid phase. In 

 the allied species Ccenurus, numerous Tsenia-heads are budded 

 off from the same cyst, each of which may originate a separate 



