162 NATURAL HISTORY 



merits of the present day. Although this is one of the 

 largest species of animalcules, yet all the authors who 

 have written upon it have given only a moderately cor- 

 rect outline of its external form, and an indistinct ac- 

 count of its internal structure : such, indeed, was all that 

 their imperfect instruments would enable them to do ; it 

 was reserved for the introduction of Achromatic Ob- 

 ject Glasses, with their large angles of aperture, whereby 

 so much additional light is transmitted, clearly to unfold 

 all the minute parts to the view of the observer. These 

 animalcules are very transparent and colourless, like 

 water; whence their name Hj/datina, and the facility 

 which they afford for observation. 



This animalcule is visible to the naked eye. The 

 largest specimens I have met with measured l-30th of an 

 inch in length, but they are more commonly found from 

 1-45 th to 1-lOOth of an inch ; so that by attentive obser- 

 vation the smallest specimens can be discerned ; for a 

 person with a good sight may perceive an object, held 

 at the shortest distance for distinct vision, of one-third of 

 that size. They are commonly found among confervae 

 and lemna. In vegetable infusions they are generally the 

 last that make their appearance, being always preceded 

 by the simpler polygastric kinds of the Infusoria, though 

 the different species of the latter do not follow any regu- 

 lar order, even in different portions of the same infusion. 

 They are broad in proportion to their length, as shewn 

 in the engraving, figure 253, which represents a speci- 

 men, l-96th of an inch in length, in its elongated con- 



