MICROSCOPIC FORMS OF ANIMAL LIFE. 53 



444. It is obvious that no Classification of Infusoria can be of any 

 permanent value, until it shall have been ascertained by the study of 

 their entire life-history, what are to be accounted really distinct forms. 

 And the differences between them, consisting chiefly in the shape of their 

 bodies, the disposition of their cilia, the possession of other locomotive 

 appendages, the position of the mouth, the presence of a distinct anal 

 orifice, and the like, are matters of such trivial importance as compared 

 with those leading features of their structure and physiology on which 

 we have been dwelling, that it does not seem desirable to attempt in this 

 place to give any detailed account of them. The life-history of the 

 ciliate Infusoria is a subject pre-eminently worthy of the attention of 

 Mitroscopists, who can scarcely be better employed than in tracing out 

 the sequence of its phenomena, with the same care and assiduity as have 

 been displayed by Messrs. Dallinger and Drysdale in the study of the 

 Monadina. " In .pursuing our researches," say these excellent observers, 

 " we have become practically convinced of what we have theoretically 

 assumed the absolute necessity for prolonged and patient observation of 

 the same forms. Two observers, independently of each other, examining 

 the same Monad, if their inquiries were not sufficiently prolonged, might, 

 with the utmost truthfulness of interpretation, assert opposite modes of 

 development. Competent optical means, careful interpretation, close 

 observation, and time, are alone capable of solving the problem." 



SECTION II. ROTIFER A, OR WHEEL- ANIMALCULES. 



445. We now coine to that higher group of Animalcules, which, in 

 point of complexity of organization, is as far removed from the preced- 

 ing, as Mosses are from the simplest Protophytes; the only point of real 

 resemblance between the two groups, in fact, being the minuteness of 

 size which is common to both, and which was long the obstacle to the 

 recognition of the comparatively elevated character of the Rotifera, as it 

 still is to the precise determination of certain points of their structure. 

 Some of the Wheel- Animalcules are inhabitants of salt water only; but 

 by far the larger proportion are found in collections of fresh water, and 

 rather in such as are free from actively decomposing matter, than in 

 those which contain organic substances in a putrescent state. Hence 

 when they present themselves in Vegetable infusions, it is usually after 

 that offensive condition which is favorable to the development of many 

 of the Infusoria has passed-away ; and they are consequently to be 

 looked-for after the disappearance of many successions (it may be) of 

 Animalcules of inferior organization. Rotifera are more abundantly 

 developed in liquids which have been long and freely exposed to the open 

 air, than in such as have been kept under shelter; certain kinds, for 

 example, are to be met with in the little pools left after rain in the 

 hollows of the lead with which the tops of houses are partly covered ; 

 and they are occasionally found in enormous numbers in cisterns which 

 are not beneath roofs or otherwise covered over. 1 They are not, how- 

 ever, absolutely confined to collections of liquid : for there are a few 

 species which can maintain their existence in damp earth ; the common 

 Rotifer is occasionally found in the interior of the leaf-celte of Sphagnum 

 ( 339); and at least two species QiNotommata also are known to be para- 

 writer to be in favor of his later statement, that the bodies figured in PI. xrv., 

 fig. 19, are really Infusorian embryos, and not parasitic Acinetae. 



1 See a remarkable instance of this in vol. i., p. 232, note. 



