NUTRITION IN THE INFUSORIA. 71 



other animals, they also must possess a stomach. But, as no 

 such structure had been rendered visible in the smallest spe- 

 cies of infusoria, such as monads, it was too hastily concluded 

 that these species were formed upon a different and a sim- 

 pler model. Lamark characterized them as bcinc;, tbrough- 

 out, of a homogeneous substaifce, destitute of mouth and di- 

 gestive cavity, and nourished simply by means of the ab- 

 sorption of particles through the external surface of their 

 bodies. 



The nature and functions of these singular beings long 

 remained involved in an obscurity which appeared to be 

 impenetrable; but at length a new light has been thrown 

 upon the subject by Professor Ehrenberg, whose researches 

 have recently disclosed fresh scenes of interest and of won- 

 der in microscopic worlds, peopled with hosts of animated 

 beings, almost infinite in number as in minuteness.* In en- 

 deavouring to render the digestive organs of the infusoria 

 more conspicuous, he hit upon the fortunate expedient of 

 supplying them with coloured food, which might communi- 

 cate its tinge to the cavities into which it passed, and exhi- 

 bit their situation and course. Obvious as this method may 

 appear, it was not till after a labour often years that Ehren- 

 berg succeeded in discovering the fittest substances, and in 

 applying them in the manner best suited to exhibit the phe- 

 nomena satisfactorily. W.e have already seen that Trembley 

 had adopted the same plan for the elucidation of the struc- 

 ture of the hydra. Gleichen also had made similar attempts 

 with regard to the infusoria; but, in consequence of his 

 having employed metallic or earthy colouring materials, 



* The results of Ehrenberg's labours were first communicated to the Ber- 

 lin Academy; they have since been published in two works in German: the 

 first of wliich appeared at Berlin in 1830, under the title of " Orga/iisatiany 

 Systematik und Geographischcs J'erhalhiiss der Infusionsthierchen." The 

 second work appeared in 1832, and is entitled " Zur Erkenntniss der Organi- 

 sation in dtr Richtung des kkinsten Baumes." Both are in folio, with plates. 

 An able analysis of the contents of the former of these works, by Dr. Gaird- 

 ner, is given in The Edinburgii New Philosophical Journal for 1831, p. 201, 

 of which I have availed myself largely in the account which follows. 



