204 NATURAL HISTORY OF BHOMPSTON POND. 



terminates in a slightly swollen end, containing the ciliated 

 mouth (d). Two contractile vacuoles are present, one at the 

 base of the neck (e) and the other near the posterior end (f). 

 This animal performs the most graceful movements as it 

 wreathes its way through the little masses of algae, at one 

 moment stretching forward its long neck, and at another 

 twisting it or jerking it out in various directions as if testing 

 the ground. 



In Amphileptus (fig. 6) the ventral mouth (a) is situated 

 between the neck-like process or proboscis (b) and the body. 

 It was present in a sample from the centre of the pond, but 

 does not appear to be plentiful. 



A still more elongated form is Spirostomum (fig. 7), which 

 seems more plentiful than the last example, and is character- 

 ised by having an adoral zone of cilia in addition to the body 

 being covered with cilia (Heterotricha). The cilia are 

 arranged spirally on the surface striae (a), and are equal in 

 length, except those above the laterally placed mouth (b), 

 which are longer. Anteriorly the long and flexible body ends 

 obtusely without a neck, posteriorly it is truncate. Spiros- 

 tomum is to be found amongst the sediment and debris of 

 the pond. 



Living parasitically within the small intestine and rectum 

 of the larvae of the newt, the writer found Balantidium 

 entozoon (fig. 8), another heterotrichous form. When these 

 parasites were fixed and then stained with picrocarmine, the 

 large single nucleus (a) was clearly displayed. The body 

 contains one or two contractile vacuoles (b) and usually many 

 food vacuoles (c), as this parasite ingests solid food taken in 

 through the peristomial opening (d), situated at the somewhat 

 pointed anterior end of the body. They swim very slowly 

 through the contents of the alimentary canal of the host ; and 

 are found in quite young larvae as well as in the more mature. 



Nyctotherus cordiformis (fig. 9) was found in association 

 with Opalina living parasitically within the intestine of the 

 frog. It differs from Balantidium in having a permanent 

 anus, but resembles it in having a definite pharynx (a), a 



