15 



and spermatozoa are developed ; in others, ovisacs and ova : 

 thus distinct males and females are produced, combining 

 the nutritive with the generative functions. The ova are 

 impregnated, are sheltered for a time in special marsupial 

 pouches, the parent also performing the part of a nurse, and 

 then the young brood issues forth under its ciliated or in- 

 fusorial form. From the polygastric type it passes into the 

 rotiferous one, from this into the polype type, and all the 

 individuals propagated by gemmation under the latter form 

 are ultimately resolved by spontaneous fission into the male 

 or female oviparous Medusae. 



I will cite a few other instances in which a species is 

 represented by a series of individuals of different powers 

 and forms succeeding each other in a cycle. Certain fresh- 

 water snails are infested by Entozoa of the order Trema- 

 toda or fluke- worms ; as the Limncea stagnalis e. g. by the 

 Distoma tarda. The ova, or products of the ova, of this 

 species are found, in early summer, adhering in vast num- 

 bers to the inner surface of the respiratory cavity and to 

 the exterior of the lobes of the liver and generative organs 

 of the snail ; where they increase in size, and detach them- 

 selves as free animalcules, assuming a bright yellow colour, 

 whence they were called by Bojanus " konigsgelben Wiir- 

 mern," and manifesting a twisting vermicular motion. If 

 one of these be microscopically examined, none of the 

 lineaments of the organs of the future Distoma can be dis- 

 cerned ; they resemble in structure rather the Gregarince, 

 consisting in fact of little else than the germ-mass, a small 

 proportion of which may have been metamorphosed to form 

 the smooth outer skin. 



As the growth of this Gregariniform parasite proceeds, 

 a progeny is seen to rise in its interior by the aggregation 

 round separate pellucid centres of the nuclear matter 

 into numerous independent germ-masses, and their de- 

 velopment into embryos : these are first manifested by 

 the appearance of an anterior and a median circular 

 sucker, and then they acquire cephalic spicula and a 



