SPECIAL PAET. 



CHAPTER VI. 

 PROTOZOA. 



Animals of single constitution and small size ; without tissues com- 

 posed of definite, cells. Sexual reproduction by means of ova and 

 spermatozoa unknown. 



From a morphological point of view the Protozoa have remained 

 at the stage of cells, in the protoplasm of which one or more nuclei 

 may be present. The phenomena of segmentation of the egg and 

 formation of the germinal layers are therefore absent from their 

 development. The body is always composed of a contractile granular 

 substance, filled with vacuoles ; it may also contain a pulsating vacuole, 

 and present the phenomenon of granule currents. The pulsating 

 vacuole consists of a space without walls filled with a clear fluid. 

 This space apparently diminishes and disappears through the contrac- 

 tion of the surrounding plasma, and then re-appears. 



There exists, however, in the varying differentiations in the 

 interior of the saroode body, and in the differences in the external 

 boundary, and in the manner of nourishment, a number of modi- 

 fications which we shall use for the foundation of groups. In the 

 simplest cases, the entire body consists of a small lump of sarcode, 

 the contractility of which is confined by no firm external membrane. 

 This lump of sarcode is sometimes semi-fluid, and protrudes and 

 retracts processes. It is sometimes of tougher consistence in parts, 

 and protrudes hair-like rays and threads (fihizopoda}. Nourishment 

 takes place through the intussusception of extraneous bodies, which 

 can be surrounded and enclosed by the protoplasmic substance at any 

 portion whatsoever of the periphery of the body. In other cases the 

 body which sends out slender processes (pseudopodia) secretes 

 silicious or calcareous needles, lattice-work shells, or shells perforated 



