INVERTEBRATE ANIMALS. 

 PROTOZOA. 



CHAPTER I. 



i. GENERAL CHARACTERS OF THE PROTOZOA. 

 2. CLASSIFICATION. 3. GREGARINIDA. 



i. General Characters. The sub-kingdom Protozoa, as the 

 name implies, includes the most lowly organised members of 

 the animal kingdom. From this circumstance it is difficult, if 

 not impossible, to give an exhaustive definition, and the fol- 

 lowing is, perhaps, as exact as the present state' of our know- 

 ledge will allow : 



The Protozoa may be defined as animals, generally of minute 

 size, composed of a nearly or altogether structureless jelly-like sub- 

 stance (termed " sarcode"}, showing no composition out of definite 

 parts or segments, having no definite body-cavity, presenting no 

 traces of a nervous system, and having either no differentiated 

 alimentary apparattis, or but a very rudimentary one. 



The Protozoa are almost exclusively aquatic in their habits, 

 and are mostly very minute, though they sometimes form 

 colonies of considerable size. They are composed of contrac- 

 tile, jelly-like protoplasm, often known by the name of " sar- 

 code," which is semi-fluid in consistence, and is composed of 

 an albuminous base with oil -globules scattered through it. 

 Granules are generally developed in the sarcode, and in many 

 cases there is a definite internal solid particle, termed the 

 " nucleus." 



In many of the Protozoa the protoplasm is not surrounded 

 by a definite outer envelope, -thus permanently remaining in 

 the condition of a mere "cytode." In other cases, how- 

 ever (Infusoria], such an outer envelope exists and a central 

 " nucleus " is present, when the organism may be compared 



