PROTOZOA INFUSORIA. 143 



example of the unattached groups. It is in shape some- 

 what like a slipper, and is covered with minute cilia, 

 which are kept constantly in motion. These cilia enable 

 the animal to move rapidly through the water, and also 

 bring a supply of minute particles of food to the mouth. 

 The outer covering of the body is a delicate membrane, 

 called the cuticle. Beneath this there is a layer of 

 somewhat firm jelly, which has been termed the cortical 

 (Lat. cortex, bark) layer. This passes gradually into a 

 semi-fluid mass which occupies the whole interior of the 

 body. No definite structure has been detected in any of 

 these layers. 



There is a funnel-shaped mouth, which is succeeded by 

 a short tube, lined with the cuticle. This tube opens 

 directly into the internal sarcode, and conveys such 

 particles of food, mixed with globules of water, as the 

 currents produced by the cilia may have brought. The 

 name Polygastrica (Gr. polus, many ; gaster, stomach), was 

 applied to the Infusoria by Ehrenberg, because he believed 

 these minute globules of water to be so many stomachs. 

 Indigestible particles pass out by an opening situated 

 near the mouth. This anal aperture does not seem to be 

 connected with any tube corresponding to an intestine. 



At one or two points there are certain clear spaces in 

 the cortical layer which open and close alternately. 

 These are denominated contractile vesicles. When 

 expanded, they appear to be filled with a clear fluid, which 

 is probably water. When contraction takes place, this 

 iluid disappears. Some observers are of opinion that 

 these vesicles communicate with the external medium, 

 and are connected with canals which radiate through the 

 central sarcode. It is probable that this apparatus is of 

 a respiratory nature, bearing some analogy to the water 

 vascular system of the Annuloida. 



A little oval mass has been observed in another part 

 of the cortical layer, to which the name nucleus has 

 been given. Besides this, there is a smaller round mass 

 which has been called the nucleolus. These are 



