192 PROTOZOA 



tide on the outside of the body, which in the 'ametabola' is firm; in the 

 others very flexible. The cuticle is covered with cilia small vibrating 

 processes which move together, and serve not only as organs of loco- 

 motion, but by creating vortices in the water bring food to the organism. 

 They furnish the most important characteristic of the class (fig. 150). 



The presence of a cuticle necessitates a cytostome, except in the para- 

 sitic species, since food particles cannot be taken in at every point. At 

 the cytostome the cuticle with its cilia forms a funnel-like food tube (cyto- 

 pharynx) into the protoplasm. At the bottom the cuticle is interrupted 

 so that water and protoplasm are in contact. By the action of the cilia 

 food particles are taken into the cytopharynx and pressed against the 

 protoplasm, forming a small enlargement which finally sinks into the 

 substance as a, food vacuole (na) which, by the streaming of the protoplasm, 

 is carried about in the body. The digestible portions are absorbed, 

 and those not -capable of digestion are cast out of the body at a fixed 

 point (cytopyge) usually not recognizable at other times (fig. 150.3). 

 Contractile vacuoles (cv) are lacking only in parasites and marine species. 

 They are constant in number and position, and frequently have afferent 

 ducts which empty into the vacuole, the vacuole in turn forcing the fluid 

 to the exterior. 



Trichocysts, nettle bodies, and muscular fibrillas occur in some species. 

 Trichocysts are minute rods vertical to the surface in the cortical layer, which 

 under the influence of reagents (chromic acid) elongate into threads penetrating 

 the cuticula. To these have been ascribed defensive functions; others regard 

 them as tactile structures. They have no connection with the cilia. Nettle 

 bodies are extremely rare. Muscle fibres lie between ectosarc and cuticle, and 

 cause quick convulsive motions of the animal. 



There are two nuclei physiologically unlike. The larger of these 

 (nucleus of older writers, macronucleus) is a large oval, rod-like, or spiral 

 body, deeply staining with microscopic stains, and surrounded with a 

 membrane. It controls all the common vital functions of the animal 

 (motion, feeding, etc.). Beside it or in a depression in it is the much 

 smaller micronudeus (nucleolus or paranucleus of older authors) which 

 stains less deeply. In all sexual processes it comes to the front and can 

 be called the sexual nucleus. 



Multiplication of Ciliata occurs by binary fission (fig. 150); more 

 rarely, and then only in the encysted condition, by division into numerous 

 parts. Budding is known in the Peritricha and Suctoria. In fission first 

 the micronucleus divides mitotically, and then the macronucleus separates 

 by elongation and constriction. The old cytostome persists in the an- 

 terior offspring, but often an outgrowth from it (2, o') passes into the 

 posterior half and develops into a new mouth. 



