The Protozoa 49 



Order Heterotrichida. Organisms possessing a uniform covering 

 of cilia over the entire body, arid an adoral zone consisting of short 

 cilia fused together into membranelles. 

 Suborder Polytrichina. Uniform covering of cilia. 



Family Bursaridm. The body is usually short and pocketlike, 

 but may be elongated. The chief characteristic is the 

 peristome, which is not a furrow, but a broad triangular area 

 deeply insunk, and ending in a point at the mouth. The 

 adoral zone is usually confined to the left peristome edge or 

 it may cross over to the right anterior edge. 

 Genus Balantidium. 



Structure. — From the table it will at once be evident that the 

 protozoa form an extremely varied group, and that no kind of 

 descriptive treatment can be looked upon as adequate that does not 

 consider individuals. 



Cytoplasm. — In some of the smaller protozoa, and in certain stages 

 of others, the cytdplasm appears almost hyaline and structureless. 

 In most cases, however, it appears granular, and in the larger organ- 

 isms, such as Ameba, it presents the appearance which some describe 

 as granular, others, as frothy. The accepted theory of structure 

 teaches that the protoplasm is honeycombed or frothy, and that it is 

 filled with endless chambers in which its enzymes and other active 

 substances, etc., are stored up and its functions carried on. 



In addition to these chambers, which are minute and of uniform 

 size, there are larger spaces called vacuoles, some of which are the 

 result of temporary conditions^ — accumulations of digested but not 

 yet assimilated food, etc.; but others, seen in Ameba and in the 

 ciliata, are large, permanent, and characterized by rhythmical 

 contractions through which they disappear from one part of the 

 body substance to appear in another. These are known as "con- 

 tractile vacuoles," and are supposed to subserve the useful purpose of 

 assisting in maintaining cytoplasmic currents and so distributing the 

 nourishing juices. 



The cytoplasm also contains remnants of undigested or indigest- 

 ible foods which constitute the paraplasm or deuteroplasm. In a 

 few cases granules of chlorophyl are also to be found in organisms 

 otherwise resembling animals too closely to be confused with plants. 



The cytoplasm may be soft and uniform in quaUty, or there may 

 be a surface differentiation into ectosarc, or body covering, and 

 endosarc, body substance. In the rhizopoda there is Httle difference 

 between the two, though certain fresh-water ameba cover themselves 

 with minute grains of mineral substance, but in most of the masti- 

 gophora and infusoria corticata the ectosarc is characterized by a 

 peculiar rigidity that gives the animal a definite and permanent 

 form. From the surface covering or ectosarc coarse threads of fine 

 hair-like appendages — -flagella and cilia — often project. In many 

 of the infusoria the ectosarc contains trichocysts from which nettling 

 or stinging threads are thrown out when the organisms are irritated. 



The body substance may show no morphologic differentiation in 

 4 



