112 THE NATURE AND VARIETIES OF 



genetic cycle, and their gemmiparous or sexual character in 

 consequence. They both, however, have this in common, 

 that the great end of their existence is the multiplication of 

 the race an end to which the nutritive and animal func- 

 tions are always subordinated. 



2. The distinctions now referred to among the cases 

 of alternation will appear more clearly by noticing the phe- 

 nomena which are actually met with in such tribes of ani- 

 mals as afford well marked examples of their occurrence. 

 The case of the Trematode Entozoa furnishes a fair illus- 

 tration of the variety in which gemmation occurs, as a 

 feature in the . progress of the development of the most 

 typical form of the species. The reproduction of these 

 Entozoa, as observed in the well-known parasite Distoma, 

 has had the attention of naturalists strongly drawn to it, 

 from being the case on which Steenstrup chiefly enlarged in 

 his original work on " Alternation of Generations." The 

 species of Distoma are very numerous ; they are bisexual, as 

 is the case with the Trematoda generally, and probably also 

 self-impregnating. The young, on escaping from the egg 

 which does not commonly happen till the ejection of the latter 

 from the system of the animal in which the parent was a pa- 

 rasite appear in the guise of infusorial animalcules, which 

 swim about freely by the play of the cilia, covering their ex- 

 terior. Some of them have been traced into the interior of 

 water snails. This infusorial form never advances to any 

 higher development; it is in fact merely the matrix of another 

 animalcule of a very different character, resembling a Gre- 

 garina, in its unifonnly granular structure and smooth con- 

 tractile integument, and the function of the original ciliated 

 envelope is mainly, as it would seem, to introduce the Gre- 

 garina into the body of the snail. This gregariniform 

 parasite itself is a mere cyst or capsule, whose gelatinous 

 walls have generally a certain degree of contractility, but 

 which never acquire more than the most rudimentary in- 



