OF THE KOTATORIA. • 659 



These he again subdivides as follows : — 



1. With elongated, transversely-ringed, attached foot. 



2. With elongated, jointed foot, retractile, like a telescope. 



3. With elongated, jointed, non -retractile foot. 

 A.<( 4. With short foot and long pedal forceps. 



o. With short foot and pedal forceps which are of equal length with, or 



somewhat shorter or longer than the foot. 

 6. Without foot. 



B. 



1. Foot short. 



2. Foot absent. 



p J rt. Depressed from above downwards ] o" -p f. -k *■' 

 [ b. Laterally compressed. 



It will be seen that the classifications of Ehrenberg, Dujardin, and Ley dig 

 agree in one featui'e : they are more or less artificial, being based upon 

 peculiarities of external fonn and habit rather than upon internal organiza- 

 tion. The subdivision of the trochal wreath varies in its extent with the 

 age of the animal, the depth of its sulci increasing with the approach of 

 maturity ; consequently the defectiveness of Ehrenberg's system becomes at 

 once obvious. No such changes as we have just referred to afi'ect the internal 

 viscera, except in a minute degree ; consequently the latter alone, when 

 thoroughly understood, can furnish the true materials for a philosophical 

 classification. But unfortunately we do not as yet possess such a number of 

 accurate observations as admit of our arranging the various species on this 

 higher basis. For example, the Rotatoria are either monoecious or dioecious : 

 a few have been demonstrated to belong to the latter class ; but of the vast 

 majority we are unable to say which of these two features characterizes 

 them. The belief in their monoecious natui^e has until recently been general ; 

 but the possibility of their being all dioecious now suggests itself. Should 

 future observations establish the fact of some Rotatoria being monoecious 

 and others dioecious, the distinction will be one of paramount importance 

 as a basis of classification. But of the internal organization of the vast 

 majority of these animals we unfortunately know little or nothing. A 

 very small number even of the higher forms have been submitted to rigid 

 and accurate scrutiny; consequently the want of material for a natural 

 classification, based on anatomical and physiological data, compels us to fall 

 back upon such as are artificial. (See Part I. p. 477 for additional remarks 

 on classification.) 



The relative value of the three systems of Ehrenberg, Dujardin, and Leydig 

 will be a disputed question. A\Tiere the pui-pose to be accomplished is merely 

 the provision of an index (and artificial systems can be little more), the clas- 

 sification in which the distinctions are most readily recognized will best fulfil 

 its purpose. On these grounds we think there is little room for choice be- 

 tween those of Ehrenberg and Dujardin. The two primarj^ sections of the 

 great Prussian natiu-alist are easily recognized, his four principal subdivisions 

 almost equally so ; and the ultimate division of each group into a loricated 

 and an illoricated series not only facilitates the investigations of the young 

 student, but is an element in Dujardin's system, who, by adopting it, re- 

 cognizes its value. At finest sight Leydig's classification would appear to 

 approach nearer to a natural system than either of the others enumerated ; 

 but close examination does not confii^m this impression, since, in order to 

 arrange the objects in their respective groups, such genera as Diglena, Far^ 



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