362 



THE INFUSORIA 



Q 



Bacteria. In all cases they move with a good deal of activity, 

 sometimes with a uniform speed and sometimes with short starts 

 and stops alternating with considerable regularity. It may be 

 noticed that in any case one end of the animal's body is habitually 

 in front, and that when it wants to change the direction of its 

 progress it turns round. This enables us to speak definitely of an 

 anterior and a posterior end as in the bilaterally symmetrical metazoa. 

 The two ends of the body are frequently so much alike in form 

 and as regards the disposition of the cilia (many species of 

 Paramoecium) that in killed and preserved specimens they are 

 almost indistinguishable. In many cases, however, the anterior 

 end may be distinguished by being more pointed (Spirostomum), by 

 the conspicuous mouth (Prorodori), by the presence of peculiar 

 sensory cilia (HYPOTRICHA), or by a special 

 collar (peristome) of long cilia on the margin 

 of a spiral ridge (HETEROTRICHA) round the 

 mouth. It is not always possible to define 

 the limits of the dorsal and ventral surfaces, 

 as the body may be almost spherical in shape, 

 and the cilia may be of approximately 

 equal length and evenly distributed on the 

 surface. In such forms, the progression of 

 the animal is accompanied by a more or less 

 rapid rotatory movement round the long axis 

 of the body. In other cases, however, the 

 body is flattened (Paramoecium) and the 

 dorsal and ventral surfaces fairly well de- 

 nned. 



Among those forms that habitually crawl 

 or creep over surfaces, the body is generally 

 oval in outline and considerably flattened 

 dorso-ventrally. A further peculiarity is 

 frequently found in the specialisation of 

 some of the cilia to serve the purpose of 

 locomotion, and perhaps also the functions 

 of touch and taste (Fig. 1, a and b). These 

 cilia are usually called cirri, and are dis- 

 tinguished by their thickened bases and con- 

 siderable length. 



Among the sedentary forms there is a 



-,.,-, P T n 



decided prevalence of more radially sym- 

 metrical shapes, with a restriction of the 

 cilia to a definite ring or series of rings 

 round the mouth. It is in the sedentary 

 forms, too, that we often find incomplete fission, leading to the 

 formation of colonies of several individuals organically connected 





FIG. 1. 



Ventral view of Oxytricha, 

 a creeping Hypotrichan. o, 

 frontal cirri, probably sensory 

 in function ; b, caudal cirri ; 



vacu le ' 



