234 Dr. C. F. J. Lachmann on the Organization of Infusoria. 



that in a vessel which at first contained a great number of Vor- 

 ticella microstoma, these in course of time gradually diminished, 

 whilst individuals of a particular species of Acineta, in this case 

 Podophrya fixa, made their appearance in constantly increasing 

 numbers. In this case changes may very probably have taken 

 place in the conditions of the surrounding medium, which were 

 unfavourable to the former species, and compelled them to become 

 encysted, whilst they probably only furnished the other species 

 with the favourable conditions for their existence and propagation. 

 A similar alternation in the occurrence of species is observed in 

 many species*, so that the admission of their relationship would 

 compel us, with Pineauf, Dr. G. Gros J, and Laurent §, to regard 

 the greater part of the Infusoria as stages in the development of 

 the same species, nay, even to place them in relationship to 

 the Rotifera, Worms, and Crustacea. In the alleged relation 

 between Vorticella microstoma and an Acineta, we also find a 

 very natural reason for the simultaneous increase of the Acineta 

 and diminution of the Vorticellce, in the fact that the latter are 

 very often sucked out by the former, frequently three to four Vor- 

 ticella at once by a single Acineta. The supposed intermediate 

 steps between the two forms of Infusoria are, as Cienkowsky || 

 has proved by direct observation, in part erroneously explained. 

 Stein's transversely costate cyst (tab. 4. fig. 30^), which he 

 supposes to have been produced from a Vorticella-cyst, and to 

 pass into a Podophrya, through the state represented in fig. 31, 

 according to Cienkowsky was rather produced through the in- 

 termediate step, fig. 31, from a Podophrya, but without becoming 

 converted into a Vorticella-cyst. 



An alternation of this kind in the appearance of particular 

 Infusoria, can only allow us to conclude that they are related, 

 when we have convinced ourselves by strict isolation that there 

 are only individuals of the one species and none of the other in 

 a particular small space, when we take care that none of these 

 can have access from without, and enable ourselves to watch the 

 individuals. This has always been neglected by Stein ; only one 

 of his observations** appears nearly to fulfil this requirement, so 



* See Schrank, Fauna Boica, iii. 2. p. 19 ; Cohn, in Siebold and Kolli- 

 ker's Zeitschr. iii. p. 258, &c. 



t Ann. des Sci. Nat. 3 serie, iii. p. 182, iv. p. 103, and ix. p. 100. 



X Ann. des Sci. Nat. 3 serie, xvii. p. 193, and various papers in the Bull, 

 de la Soc. Imp. de Naturalistes de Moscou. 



§ Various memoirs in the Memoires de la Soc. des Sciences, &c. de 

 Nancy, and Etudes Physiol, sur les Animalcules des Infusions Vegetales, 

 1854. 



II Bull, de I'Acad. de St. Petersb. 1855, p. 297. 



51 Orcula Trochus of Weisse. 



** L. c. supra, p. 39. 



