36 LECTURE n. 



any other organ properly so called, it reduces our definition of an 

 animal to the difference indicated in the preceding comparison. 



The Rhizopoda are but little better organised, although they are 

 locomotive, and provided with long slender branched filamentary feet, 

 and have their simple contractile tissue protected, in most species, by 

 chambered foraminiferous shells. 



The ciliated Polygastria exhibit the next step in the progress 

 of individualising a higher independent embodiment of animal life. 

 A firm central nucleus in which, as in Gregarina and Amcsba, resides 

 the mysterious property of spontaneous division, indicates, however, 

 their essential character as animated cells, and recalls to mind the bold 

 figure in which Oken long since indulged, when he afiirmed that the 

 higher animals, and even Man himself, were aggregates of Infusoria. 

 The very step, however, which the Infusoria take beyond the primi- 

 tive astomatous cell-stage of their existence, invests them with a spe- 

 cific character, as independent and distinct in its nature as that of the 

 highest and most complicated organisms. No mere organic cell, 

 destined for ulterior changes in a living body, has a mouth armed 

 with teeth, cavities for digestion, pulsatile cells for circulating a clear 

 plasmatic fluid, an irritable and contractile integument beset with 

 vibratile cilia, or prolonged into tentacnla. 



And now you may be disposed to ask : To what end is this discourse 

 on the anatomy of beings too minute for ordinary vision, and of 

 whose very existence we should be ignorant unless it were revealed 

 to us by a powerful microscope ? What part in nature can such 

 apparently insignificant animalcules play, that can in any way interest 

 us in their organisation, or repay us for the pains of acquiring a know- 

 ledge of it ? I shall endeavour briefly to answer these questions. 

 The polygastric Infusoria, notwithstanding their extreme minuteness, 

 take a great share in important ofiices of the economy of nature, on 

 which our own well-being more or less immediately depends. 



Consider their incredible numbers, their universal distribution, 

 their insatiable voracity ; and that it is the particles of decaying 

 vegetable and animal bodies which they are appointed to devour and 

 assimilate. 



Surely we must in some degree be indebted to these ever active 

 scavengers for the salubrity of our atmosphere. Nor is this all : 

 they perform a still more important office, in preventing the pro- 

 gressive diminution of the present amount of organised matter upon 

 the earth. For when this matter is dissolved or suspended in water, 

 in that state of comminution and decay which immediately precedes 

 its final decomposition into the elementary gases, and its consequent 

 return from the organic to the inorganic world, these wakeful 



