92 THE OCEAN WORLD. 



indeed been found possible to observe the organisation of the diges- 

 tive system in a satisfactory manner. Some writers think they have 

 no mouth, what has been taken for that organ being only a hollow 

 dimple on the surface of the body ; others recognise the existence of 

 an oral orifice or mouth, sometimes specially furnished. 



The digestive system is better understood in the superior Infusoria, 

 called the ciliate hifusoria, namely, those provided with vibratile cilia. 

 These cilia seem to determine the currents which convey the numerous 

 nutritive corpuscles suspended in the water towards the entrance of 

 the digestive system. They form, in some sort, the prehensile organs 

 which seize the aliment. 



The minute particles of food thus directed towards the oral orifice 

 by the vibratile cilia are soon engulfed by the mouth and speedily 

 disappear into the interior of the animal. Availing himself of this 

 fact, and of the transparency of these creatures, Gleichen, a German 

 physiologist of the last century, conceived the happy idea of colouring 

 the water which contained Infusoria with finely-powdered carmine, 

 and he thus traced the colouring matter into the bodies of some of 

 them. But it was reserved for Ehrenberg to avail himself of the 

 same artifice in order to study more in detail the internal structure 

 of these minute creatures, as well as their mode of absorbing nutritive 

 matter. This physiologist fed many groups of Infusoria, some of 

 them with water coloured with carmine, others with indigo and other 

 colouring matters ; the coloured particles were greedily swallowed, 

 and were thought to show the arrangement and disposition of the 

 alimentary system, for he arrived at the conclusion that, as the colour- 

 ing matter was deposited in apparently perfect cavities, so each of 

 these cavities was a stomach, and that the passage of the food into 

 each of these reservoirs was effected by means of an intestinal tube, 

 around which these stomachs were arranged. In some cases he even 

 thought he could distinguish, and again in others he figured, the out- 

 lines of this intestinal canal, and showed its connection with numbers 

 of the little bladder-like stomachs. We now know that his class 

 Infusoria embraced two very different forms of animal life, the Poly- 

 gastrica and Rotifera, the latter division including the well-known 

 Wheel animalcules ; the Polygastrica being so called from his idea 

 that the typical forms possessed a number of stomachs. That 

 Ehrenberg recognised a difference between them is apparent from 

 his division ; but the organisation of the Rotifera remove them very 

 far indeed from the true Infusoria. 



Other observers were not slow in raising objections to these views 

 of Ehrenberg. Dujardin, especially, did not believe in the complex 



