INFUSORIA. 181 



are subdivided in two principal groups, infusoria and 

 rhizopods. 



The infusoria are little masses of definite form, com- 

 posed of protoplasm or gelatinous living matter. They 

 are covered with an envelope carrying vibratile cilia. Of 

 all the protozoa they are the most highly organized 

 beings, for they 



possess a digestive FIG. 109. 



apparatus with 

 two orifices, and 

 in their mass we 

 can distinguish 

 vacuoles or clear 

 spaces, and a pul- 

 satile vesicle which 

 probably serves for the digestion and circulation of a 

 nutritive liquid. These little beings have been named 

 infusoria- because their first observers found them in 

 putrid infusions of vegetables ; they are especially abun- 

 dant in stagnant waters. 



Their reproduction takes place by eggs and also by 

 scission, which is simply a division of the creature in 

 two. 



They are divided into several groups, according to the 

 arrangement of the cilia and the general form of the 

 body. 



1. The cilia may be arranged in longitudinal lines 

 covering the entire body excepting around the mouth. 

 Among the forms of this kind are paramecium and 

 opalina, 



2. The body may be covered with cilia, with long rigid 

 hairs around the mouth. The stentor and bursaria are 

 examples. 



16 



