PROTOZOA 



45 



objects, often by means of a stalk, which may or may not be 

 contractile. Among the attached species are some that live in 

 colonies. The body is always bounded by a delicate membrane 

 or cuticula, within which the protoplasm is clearly differentiated 

 into an ectosarc and entosarc. In a few cases a chitinous shell 

 is developed, but this is extremely rare (Fig. 27). Cilia project 

 beyond the surface of the body and vary greatly in number 

 and distribution, occurring frequently in longitudinal bands ; 

 we shall consider their arrangement under the several orders. 

 The ectosarc is more or less modified, often highly so, and may 

 contain fine contractile threads, which allow the animal to alter 

 its general form. In a few cases, occurring 

 chiefly in the first order mentioned below, 

 minute capsules lie in the ectosarc, each con- 

 taining a delicate, coiled thread, which may 

 be projected when the animal is irritated ; these 

 structures are known as trichocysts. In the 

 granular entosarc lies the nucleus, which pre- 

 sents the most varied shapes, — spherical, ellip- 

 soid, ovoid, elongated, and straight or wavy, 

 U-shaped, moniliform, or very irregular. This 



nucleus is often spoken of as the macronucleus, 



g a 1 1 ii LDicfyocysfo 



since we find nearly always one or more smaller ' ' 



' . . fig. 27. shell of 



nuclear structures lying near it, and these are oh one of 



called micronuclei. There may be one, two, or the Clliata - (From 



J Butschli's Protozoa.) 



many contractile vacuoles, and there are fre- 

 quently one or more contractile radial canals, which empty 

 into them. Pigment granules are often present in the entosarc 

 and sometimes chlorophyll granules. There is generally a 

 permanent mouth in all the Ciliata where the cilia are present 

 throughout life, and this leads into an oesophagus. The undi- 

 gested food particles pass off near the posterior end of the body. 

 The most common method of reproduction is by transverse 

 division ; in some species budding occurs and in some spore 

 formation. In some cases, after reproduction by division has 

 gone on for some time, a temporary union of two individuals 

 takes place, accompanied by an interchange of micron uclear 

 substance. After this conjugation, the two individuals separate 

 and reproduction by division continues. The micronucleus 



