THE INFUSORIA 



431 



pendently during the asexual cycle, with the cell as its environment. 

 In contrast to the extreme elaboration in the structure of the indi- 

 vidual, the life-cycle as a whole is generally of 

 a simple type, and the majority of the free- 

 living species are practically monomorphic ; 

 but some of the parasitic forms show a 

 succession of form-changes in their life-cycle. 



Habits, Mode of Life. The majority of 

 Ciliata are free-living aquatic forms, marine 

 or fresh-water, probably without exception 

 hglozoic in the mode of nutrition ; but a 

 great number of parasitic forms are known. 

 A ciliate, whatever its mode of life, may be 

 free or sedentary. The free forms may be 

 of swimming or creeping habit, using their 

 cilia in the one case to move freely through 

 the water or to glide along firm surfaces, in 

 the other to creep over solid objects or on 

 the surface film of the water. The sedentary 

 forms may be attached temporarily or more 

 or less permanently to some object, which is 

 often the body of some larger animal. Para- 

 sitic forms may be, as in other cases, epizoic 



or entozoic ; but the word " parasitic " must 118$?%$)I- N 



be taken in a wide sense, since many Ciliata 

 living in or upon other organisms are not 

 parasitic in the strict sense of the word, 

 though many truly parasitic forms occur. 



Body-Form. Correlated with the diversity 

 in the habit of life, the body-form and 

 external structure show many variations. 

 The primitive type of ciliate may be con- 

 sidered to be an ovoid, gooseberry-shaped 

 organism with a principal axis parallel to the 

 direction of movement, consequently with an 



Fia. 180. Spirostomum ambtguum, one of the largest 

 free-living Ciliata, reaching a length of 3 millimetres, 

 consequently a favourable object for physiological 

 experiments. N] Macronucleus, greatly elongated, 

 in shape like a string of beads or sausages (so-called 

 " moniliform " type) ; o, mouth at the hinder end of 

 the elongated peristome ; c.t., contractile vacuole, 

 supplied by a very long feeding-canal (f.c.) ; the 

 micronucleus is not shown. After Stein. 



anterior and a posterior pole (Fig. 14, p. 32) The mouth is terminal 

 at the anterior pole. The cili^ clothe the whole body evenly, being 



