22 LECTURE II. 



teristic arrangement which Ehrenberg has described and figured in 

 particular species ; as, for example, in the Vorticella, a circular 

 arrangement, or the wavy disposition in Leucophrys : yet such a 

 constancy in the arrangement of the assimilative sacs in these genera 

 is the result of my experience. Add to this, if they have not orifices 

 of communication with the alimentary tract, the difficulty of account- 

 ing for the rapid and ready transmission of the coloured aliment 

 into their interior without the surrounding parenchyme being stained. 



It is possible that, besides digestive organs, the Polygastria may 

 have a vascular system for the conveyance of the assimilated fluid 

 throughout their frame; their minuteness ought to be no objection 

 to such a conjecture, for it is merely a relative idea. Probably the 

 reticular markings on the superficies of certain species may indicate 

 such vessels: it is certain that here is placed that mechanism for 

 renewing the surrounding oxygenised medium upon that surface, 

 which we find to be the essential respiratory dynamic of the gills in 

 most of the molluscous animals. At all events, to no other part of the 

 pbtygastric organism than to this ciliated superficies can the respiratory 

 function be attributed. But the action of the vibratile cilia upon 

 the water is necessarily attended in the free Infusoria with a reaction 

 which rolls the little animalcule through its native element and 

 produces the semblance of a definite voluntary movement. 



Perhaps the most marvellous part of the organisation and economy 

 of the Polygastric Infusoria is that which relates to the function of 

 generation. This function, I may observe, is the only one which does 

 not necessarily require a special organ for its performance. I am 

 not aware that this proposition has been before enunciated, but it 

 will be quite intelligible when the essential nature of the generative 

 process is better understood. 



Although both ovaria and testes have been unequivocally de- 

 monstrated in the Polygastria^ yet their most common mode of 

 propagation is quite independent of, and superadded to, the function 

 of these organs. In a well fed Monas, Leucophrys, Enchelys, or 

 Paramecium, the globular parenchyme may be observed to become a 

 little more opake and apparently more minutely subdivided : then a 

 clear line may be discerned stretching itself transversely across the 

 middle of the body and indicating a separation of the contents into 

 two distinct parts. The containing integument next begins to con- 

 tract along this line, and the creature to assume the form of an hour- 

 glass {fig, 11.) : this, tliough doubtless an uncontrollable, seems to be 

 a spontaneous action, and the struggle of each division to separate 

 itself from its fellow indicates an impulse in each to assume its 

 individual and independent character; the which they no sooner effect 



