Monadina.] the infusoria. 87 



Family I— MONADINA. 



The animalcules of the family Monadina are the most 

 minute living creatures which have been discovered by- 

 man. They are destitute of an alimentary canal ; are 

 illoricated or shell-less, and have an uniformity of body 

 without any appendages issuing from it, cilia not being 

 considered as such. They increase either by a simple and 

 complete self-division of the body into two parts, or by 

 other sections^ into four or more individuals. The uni- 

 formity or unvarying appearance in their external forms 

 may be considered as one of the principal characteristics 

 of this family ; for no Monadina can vokmtarily alter the 

 shape of its body, whether into a filiform, knotty, or 

 globular figure, nor can it extend any portion of it, and 

 then contract it again. It is quite evident that they all 

 possess organs of locomotion, nutrition, and propagation, 

 which latter are of the hermaphrodite character. Some 

 of them have a rudimentary eye, but it has never been 

 discerned that they are furnished vrith a vascular or circu- 

 lating system, which, however, is not surprising, when we 

 reflect that should they possess it (a supposition by no 

 means to be rejected), the diameters of the tubes of this 

 system would necessarily be of such extreme minuteness 

 as hitherto to have defied investigation. 



The groups and figures in Plate I., numbered 1 to 20, 

 convey a very fair idea of the appearance of the Monadina. 

 None but microscopes of high magnifying powers can 

 develope their structures ; indeed, they cannot be ob- 

 served accurately with a less amplification than 500 



