NATURAL HISTORY OF BHOMPSTON POND. 201 



aquatic plants minute organisms disport themselves, swim- 

 ming most gracefully, or darting hither and thither in all 

 directions in countless myriads, while in the sediment, or on 

 submerged supports, others, in even greater number and 

 variety, can easily be detected hunting for their food. 



In this essay a brief description of the simpler forms leads 

 on to that of the most complex ; those forms whose structure 

 or functions seem especially interesting or instructive are 

 dealt with as fully as space permits, while the closely allied 

 forms are briefly noticed. Only those forms of life are 

 mentioned which are now living in the pond (Part 1), or 

 in the immediate vicinity (Part 2). 



PART 1. 

 ZOOLOGY. 



The vast majority of the tiny specks of animal life which 

 the microscope reveals are unicellular, their protoplasm 

 being either free to form pseudopodia (Gymnomyxa), or is 

 contained in a limiting membrane, the cuticle, giving the 

 animal a definite shape (Corticata). 



Arcella (Plate 1, fig. 1) is the most plentiful representative 

 of the former class and is to be found chiefly on the mud. 

 By accurate focussing with a ^ lens the movements of the 

 pseudopodia and even the ingestion of diatoms were distinctly 

 observed (fig. 1, a). The animal consists of a protoplasmic 

 cell containing nuclei, food vacuoles (b), contractile vacuoles, 

 and often a gas vacuole acting as a hydrostatic balance, 

 the whole cell being practically enclosed in a brownish 

 hemispherical chitinous shell (c), convex above, flat and 

 perforated with a single aperture (d), for the egress of the 

 pseudopodia below. This animal moves extremely slowly in 

 the direction of the most protruding pseudopodia (e), about 

 three of which usually project beyond the edge of the shell. 



