50 



GENERAL ORGANIZATION OF THE PROTOZOA 



the largest forms of protozoa, the eiha are distributed evenly over the 

 entire cell body. But in some cases they are limited to one-half of 

 the body, as in halteria; in others to the ventral surface only, as in 

 gastrostyla, oxytricha, and the hypotrichida in general, while in others 

 they are reduced to a single girdle of cilia about the mouth, as in 

 vorticella and its allies. 



An interesting feature in the comparative anatomy of infusoria is 

 the fusion of simple cilia into motile organs of a more complicated 

 type. Sometimes a bundle of cilia are grouped together in a small 

 brush-like organ, as in aspidisca, where the constituent elements of 

 the bundle can still be made out (Fig. 17). In other forms, as 

 oxytricha, the bundles are more tightly fused to form compact motile 

 organs, which are sometimes used for walking and running, or some- 

 times they are differentiated for feeling, and so constitute an elemen- 



FiG. 18 



Pleuronema chrysalis, Ehr., with well-developed undulating membrane. (After Calkins.) 



tary sensory apparatus. Again, the cilia are fused into continuous 

 sheets, or membranes, which provide currents for bringing food toward 

 the mouth, as in pleuronema or lembus (Fig. IS). 



Rows of small membranes, called membranelles, are found in three 

 of the four orders of ciliata. These are always placed around the oral 

 or peristomial cavity, and their synchronous beating brings a constant 

 food-bearing current toward the mouth. In some cases, as the vor- 

 ticella group, the cilia have quite disappeared, leaving, under ordinary 

 vegetative conditions, only this row of membranelles. 



In one subdivision of the infusoria, the suctoria, the cilia disappear 

 after a short embryonic life of the individual, and their place is taken 

 by protoplasmic prolongations called tentacles. Some of these ten- 

 tacles are hollow and provided with a suction cap, so that food mav be 

 drawn through them into the inner protoplasm. Others are sharp 



