208 NATURAL HISTORY OF BHOMPSTON POND. 



terminates anteriorly in a collar-less tractellum (a) (Lisso- 

 flagellata), which arises from the pharyngeal wall (b). Behind 

 the pharynx is the contractile vacuole (c) and a bright red 

 pigment spot (d), which is possibly sensitive to light. Staining 

 reveals a single central nucleus (e). Multiplication by 

 longitudinal fission can sometimes be observed. The cell 

 exhibits characteristic rhythmical changes in shape known as 

 Euglenoid movement. The elongated body shortens and 

 thickens anteriorly (fig. 19), then the enlarged part, gradually 

 lessening, appears to pass towards the posterior end, where it 

 becomes lost to view. These changes of form are then repeated. 



In marked contrast to this species is Euglena longicauda 

 (fig. 20), with a flattened oval body, green with longitudinal 

 striations and a long colourless tail as long as the body. 



A third species was also found in the pond, namely, Euglena 

 pleuronectes (fig. 21), which differs from the last in having a 

 tail only about J the length of the body. The striations 

 are less marked and the green body is more circular. It is 

 about 1 -600th in. in length. The last two species do not exhibit 

 Euglenoid movement. 



Those extraordinary living forms known as Rotifers or 

 Wheel Animalcules (Rotifera) are very abundant both in 

 numbers and species ; some swim by the action of the retractle 

 ciliated disk, the trochus, which gives the well-known wheel 

 effect, the foot then acting as a rudder. Others move in a 

 leech-like fashion by attaching the ciliated end of the body 

 and then drawing up the foot, which in many species is 

 telescopically retractile and furnished with a forked extremity 

 (toes). The body is covered with a firm integument and often 

 with a chitinous shield, the lorica. When the animal is 

 fixed the currents set up by the trochus wash food particles 

 into the mouth which leads through the ciliated oesophagus 

 to the so-called mastax (fig. 22, d) containing the trophi 

 (two mallei and an incus), the extraordinary movements of 

 which can easily be watched under a high power. Perhaps 

 the most striking example is Noteus quadricornis (fig. 22), 

 a form whose internal structure can be seen through the 



