244 FOURTH REPORT — 1834. 



plying them with organic colouring matter as nutriment, he has 

 clearly ascertained that they are not mere homogeneous gela- 

 tinous masses supported by cutaneous absorption, as was for- 

 merly supposed, but organized bodies, provided in all cases with 

 at least a mouth and digestive system. This last indeed he 

 has found subject to great variation of structure, being some- 

 times simply a round sac in the centre of the body, at other 

 times a long canal, often very much convoluted, and furnished 

 with a great number of caecal appendages, which he considers 

 as so many distinct stomachs. The mouth also varies in its 

 structure, and presents good characters for distinguishing the 

 subordinate groups. In the simpler Infusoria, it is a mere 

 xmarmed opening, surrounded with a greater or less number of 

 ciliae. In those of a higher order, however, it is much more 

 complicated, and in some cases even provided with a distinct 

 pair of serrated mandibles. Besides a digestive apparatus, Ehren- 

 berg has discovered a generative, and often a muscular system, 

 and has even in one or two instances observed traces of what 

 he considers as vascular and nervous systems. The existence 

 of these last, however, is at present somewhat problematical. 



These striking discoveries have naturally led Ehrenberg to 

 reject entirely the principles upon which all former classifica- 

 tions of these animals had been grounded, and to construct a 

 new one after the internal organization. His arrangement is 

 based upon the structure of the digestive system, which gives 

 rise to the two natural classes of Polygastrica and Rotatoria ; 

 the former consisting of sucli as are provided with several sto- 

 machs or internal cavities ; the latter of such as have only one, 

 the moutli at the same time being surrounded by a peculiar 

 rotatory apparatus. Of these two classes the last is much more 

 complex in its structure than the former. It would even seem 

 to be more highly organized than some other classes in the 

 system, to which the animals included in it have been hitherto 

 always thought subordinate. With respect to the inferior groups, 

 those of the Polygastrica are characterized from the presence 

 or absence of an excretory orifice, the relative positions of the 

 mouth and anus when this last is present, and from the presence 

 and situation of the cilise and other processes : in the Rotatoria, 

 the families are characterized from the mode of arrangement of 

 thecili{fi which form the rotatory organ. In each class the ge- 

 nera form two parallel series, one consisting of the naked Infu- 

 soria {Niida, Ehrenb.), the other of such as are protected by a 

 crustaceous or horny covering {Loricata, Ehrenb.), these two 

 series appearing to be intimately allied, and often presenting no 

 other difference beyond that which has been just alluded to. 



