364 



THE INFUSORIA 



exhibited in this respect by many parasites belonging to other 

 classes of the animal kingdom ; but although the entozoic Infusoria 

 are frequently called parasites, it is doubtful whether this term is 

 correctly applied to them. The general conception of a parasite 

 is an organism that gets its food and protection at the expense of 

 its host, and that when present in considerable numbers causes 

 injury or weakness. 



There is no evidence at present that the Infusoria feed upon 

 anything except substances that would, without their presence, be 

 ejected from the body of the host ; and it is quite possible that by 

 disintegrating certain substances in the alimentary canal, they may 

 be advantageous rather than deleterious to their hosts. The structure 

 of the entozoic Infusoria is so varied that it is usually impossible to 

 recognise in them any special adaptation to their environment. 

 The mouthless Opalina found in the bladder of frogs may owe its 

 many peculiarities of form to entozoic habits, but there is nothing 

 of importance in the structure of Nyctotherus, occurring in the 

 rectum of the same animal, that can be regarded as associated with 

 its peculiar habitat. 



Some groups of animals appear to afford much more favourable 

 conditions for the entozoic Infusoria than others. They are rarely 

 absent from the rectum and bladder of adult 

 Amphibia, the intestines of white ants and 

 other Orthoptera are frequently crowded 

 with Trichonymphidae, and the alimentary 

 canal of cows, horses, and other herbivorous 

 mammalia support a large number of remark- 

 able genera. In the coecum of the horse, 

 Bundle has found no less than thirteen dis- 

 tinct species. (Cycloposthium,, Blepharocorys 

 (3 species), Fig. 4, Paraisotricha (3 species), 

 Didesmis (2 species), Biitschlia, Blepharopros- 

 thium, Blepharosphaem, and Blepharocodon.} 

 They have not been found in the Carni- 

 vora. In the intestines of man, Balantidium 

 minutum and Nyctotherus faba occasionally 

 occur. 



SIZE. The size varies considerably. 

 Some of the elongated forms, such as Spiro- 



~ mm - 



appendage. wnen f u \\y expanded, but the more compact 



oval or rounded forms rarely reach a greater 

 size than I'o mm. in their longest axis (e.g. Bursaria). The usual 

 size is between this and 0'04 mm. in length (e.g. Cinotochilum). 



The substance of the body is clearly differentiated into an 

 outer, more solid cortical sheath and an inner, semifluid 



4-M 



FIG. 4. 



Blepharocorys uncinata, Fior., 

 from the coecum of the horse. 

 M, meganucleus ; m, micronu- 

 clei ; C.V., contractile vacuole ; 

 A, anus surrounded by cilia ; 

 C, mouth ; h, hood overhanging , . f n 



the mouth, bearing many long stomum, may attain to a length of 3 



