NATURAL HISTORY OF BHOMPSTON POND. 223 



watched in the tail of a living specimen. These active little 

 creatures feed on entomostraca, &c. ; the writer found one 

 hundred and ten water-fleas in the stomach of a single larva. 

 Water beetles, in turn, prey upon these larvae. 



The Frog (Rana temporaria) was comparatively scarce at 

 the pond during the summer months, only a few specimens 

 being obtainable. The internal structure differs from that 

 of the newt in several important points, such as the skeleton, 

 heart, lungs, reproductive organs, arterial arches, and venous 

 systems ; while the absence of tail (Anura) and other external 

 differences are obvious enough. The tongue, being free 

 behind, is attached anteriorly to the floor of the mouth, an 

 arrangement which enables it to jerk out its tongue and 

 catch insects at some distance. The markings in the skin 

 are due to dark branched pigment cells (fig. 77). The frog is 

 a host of many parasites. In the lungs of one small frog there 

 were ten trematodes (see above), while in its intestine a 

 nematode and several hundred Opalina, Nyctotherus, and 

 Balantidium were found. 



BOTANY. 



There are fewer plants than animals actually in the pond, 

 though the pond-side abounds with vegetation. A few 

 specimens of Nostoc (fig. 79) were discovered, the cells of 

 which form a filament surrounded by a colourless gelatinous 

 sheath secreted by the cell wall. These plants are closely 

 allied to Bacteria, but the absence of cilia and of endospores 

 are distinguishing features. Reproduction takes place by 

 cell division ; the filament breaks across at one of the larger 

 cells known as heterocysts (a), the detached portion or 

 hormogonia then starts a new colony. 



Tolypothrix (fig. 80), with its blue-green filament (a), 

 surrounded by a mucilaginous sheath (b), occurs frequently 

 together with the closely allied Oscillaria (fig. 81). These 

 plants seem to be constantly moving the end of the filament 

 from side to side. 



