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 ci/iate Infusoria, 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 Motifera 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 
