TENACITY OF LIFE. — CLASSIFICATION. 477 



physiological analogy than that of Mr. Huxley ; since, in the other 

 instances referred-to, as in the Rotifera, the multiplication by 

 Gemmation goes-on rapidly so long as food and warmth are abun- 

 dantly supplied ; but gives place to the Generative process, when 

 the nutritive activity is lowered by their withdrawal. 



362. Certain Rotifera, among them the common Wheel-Ani- 

 malcule, are remarkable for their tenacity of life, even when 

 reduced to the state of most complete dryness ; for they can be 

 kept in this condition for any length of time, and will yet revive 

 very speedily upon being moistened. Experiments have been carried 

 still farther with the tribe of Tardigrada (§ 363, IV. ); individuals 

 of which have been kept in a vacuum for thirty days, with sulphuric 

 acid and chloride of calcium (thus suffering the most complete 

 desiccation that the Chemist can effect), and yet have not lost 

 their capability of revivification. This fact, taken in connection 

 with the. extraordinary rate of increase mentioned in the preceding 

 paragraph, removes all difficulty in accounting for the extent of 

 the diffusion of these animals, and for their occurrence in incal- 

 culable numbers in situations where, a few clays previously, none 

 were known to exist. For their entire bodies may be wafted in a 

 dry state by the atmosphere from place to place ; and their return 

 to a state of active life, after a desiccation of unlimited duration, 

 may take place whenever they meet with the requisite conditions 

 — moisture, warmth, and food. It is probable that the Ova are 

 capable of sustaining treatment even more severe than the fully 

 developed Animals can bear ; and that the race is frequently con- 

 tinued by them when the latter have perished. 



363. The principles on which the various forms that belong to 

 this Class should be systematically arranged, have not yet been 

 satisfactorily determined. By Prof. Ehrenberg, the disposition of 

 the ciliated lobes or wheel-organs, and the enclosure or non-enclo- 

 sure of the body in a lorica or case, were taken as the basis of his 

 classification ; but as his ideas on both these points are incon- 

 sistent with the actual facts of organization, the arrangement 

 founded upon them cannot be received. Another division of the 

 class has been propounded by M. Dujardin, which is based on the 

 several modes of life of the most characteristic forms. And in a 

 third, more recently put forth by Prof. Leydig, the general con- 

 figuration of the body, with the presence, absence, and conforma- 

 tion of the foot (or tail) are made to furnish the characters of the 

 subordinate groups. Either of the two latter is certainly more 

 natural than the first, as bringing together for the most part the 

 forms which most agree in general organization, and separating 

 those which differ ; and we shall adopt that of M. Dujardin as 

 most suitable to our present purpose. 



i. The first group includes those that habitually live attached by 

 the Foot, which is prolonged into a pedicle ; and it includes two 

 families, iheFloscidarians and the Melicertians, both of which bear 



