ANIMAL PARASITES 93 



PRACTICAL WORK 



Open a freshly killed frog and mount in a drop of physiological 

 salt solution (0.6 per cent NaCl) a bit of the black fecal content 

 of the rectum. The infusoria are so large that they can be seen 

 with the naked eye as actively moving, minute specks. Of these 

 the more opaque, oval forms will probably prove to be Nyctotherus 

 cordiformis, which we shall take as a type. 



Nyctotherus cordiformis (Fig. 2) is a heterotrichous form; that 

 is, it possesses two types of cilia. Covering the entire cell are 

 parallel rows of short, similar cilia; but in addition there is an 

 "adoral zone" of large, coarse cilia, or membranelles, which form 

 a peristome extending from the anterior end along the side to 

 the mouth, or cytostome, and into the so-called cytopharynx. 

 Note the movement of these cilia and of those covering the 

 general body surface. Near the posterior end is a single con- 

 tractile vacuole and at the end a cytopyge through which excreta 

 are discharged. In stained preparations note the large bean- 

 shaped macronucleus and close to it, on the hinder concave side, 

 the small micronucleus. Make a drawing showing the above 

 features. 



Balantidium entozoon (Fig. 2) is less common than Nyctotherus 

 cordiformis in our native frogs but may be found. It is smaller 

 and may be distinguished by the smaller peristome which is 

 at the anterior end instead of extending halfway down the side. 

 The important Balantidium coli of man and of the pig will be 

 studied as a type of the genus. 



Opalina (Fig. 31) species are the most common of the ciliates 

 of frogs and toads. They are large, much-flattened Protozoa 

 with a uniform covering of cilia and without a cytostome. The 

 most characteristic feature is the presence in the adult speci- 

 mens of many nuclei of a uniform type. Reproduction in the 

 adult state is by oblique division, but during the spring months 

 there is adaptation to parasitic life in the form of the production of 

 numerous minute individuals which encyst and pass out with the 

 feces of the frog. These cysts are taken up by tadpoles and 

 liberate microgametocytes and macrogametocytes in the rectum 

 of their new host. These cells give rise to gametes which conju- 

 gate, forming zygotes which develop into the multi-nucleate 

 opalinid. 



Study motile and stained preparations of the stages available. 



