494 INFUSORIA. 



Every one knows the 



S. officinalis, or common sponge, which is found in large brown masses, 

 formed of extremely fine, flexible, and elastic fibres, perforated with numerous 

 pores and little irregular canals, all of which intercommunicate. 



CLASS V. 

 INFUSORIA. 



NATURALISTS usually close the catalogue of the animal kingdom 

 with beings so extremely minute as to be invisible to the naked eye, 

 and which have only been discovered since the invention of the micro- 

 scope has unveiled to us, as it were, a new world. Most of them 

 present a gelatinous body of the greatest simplicity, and for these, 

 this is undoubtedly the situation ; but authors have placed among the 

 Infusoria, animals apparently much more complicated, and which only 

 resemble them in their minuteness, and the dwelling in which they are 

 usually found. 



They will constitute our first order, though we must still insist 

 upon the doubts relative to their organisation, which are not yet 

 dissipated. 



ORDER I. 



ROTIFERA. 



THE Rotiferac, as above stated, are distinguished by a greater degree of 

 complication. Their body is oval and gelatinous; we can 

 distinguish in it a mouth, a stomach, an intestine, and an 

 anus near the first. It most commonly terminates posteriorly 

 in a tail that is variously constructed, and anteriorly it bears a 

 singular organ, variously lobate, with denticulated edges, and 

 of which the denticulations vibrate successively in such a 

 manner as to give the organ itself the appearance of one or 

 more dentated and revolving wheels. One or two prominences 

 on the neck have even appeared to some observers to be 

 furnished with eyes. This revolving organ does not serve to 

 direct their aliment to the mouth ; it may be supposed to have 

 some connection with the function of respiration. In 



