OF ANIMALCULES. 89 



advances in growth, alters its form so entirely, that even 

 by an observer well acquainted with other mature ani- 

 malcules, the young cannot be identified ; for although 

 its peculiar form renders it easy to be distinguished when 

 full grown, yet the infant specimen may be readily mis- 

 taken for a Monad, This circumstance cannot be too 

 strongly noticed ; for unless we are constantly on our 

 guard we shall often be led to consider the same ani- 

 malcules at different stages of their growth as so many 

 distinct species. But in order to illustrate this point more 

 fully, I shall refer to a series of figures, by Dr. Ehrenberg, 

 representing this animalcule in its various states of de- 

 velopment, to give the reader a clear idea of its characters. 

 The drawings are all magnified 380 times linear. 



The normal form of this species, when fully developed, 

 is shewn at figure 80. Its general contour has some 

 resemblance to a bean; and if we suppose the two convex 

 lobes on the front side equal, and do not perceive the 

 proboscis, which indeed is seen with difficulty, we shall 

 distinguish some likeness to the bosom; from which 

 similitude it has received its name. The mouth, which 

 is lighter tinted than the surrounding parts, is situated 

 in the hollow between the upper lobe and the proboscis, 

 and marked by a cross, while the termination of the ali- 

 mentary canal is in the cavity immediately below the 

 proboscis. The margin of the two lobes, as shewn in the 

 drawing, is furnished with a row of delicate cilia, only 

 observable imder favourable illumination. These cilia, 

 by producing a current in the water towards the mouth, 



H 



