in. CILIATA. 



205 



species; this in the ' ametabolous' forms is unalterable, in the 

 ' metabola ' it can be pressed out of shajie in passing through a 

 narrow space. This constancy of form is due to the development 

 of more or less cuticle on the outside of the body, which in the 

 ' ametabola ' acquires an armor-like firmness; in the others is more 

 flexible. The cuticle is covered with cilia — small vibrating j)ro- 

 cesses which move not singly but together in large numbers, and 

 serve not only as organs of locomotion, but by 

 creating vortices in the water bring food to the 

 organism. They furnish the most important 

 characteristic of the class (fig. 144). 



The presence of a cuticle necessitates a 

 cytostome (except in the parasitic species), 

 since food particles cannot be taken in at any 

 point. At the cytostome the cuticle with its 

 cilia forms a funnel-like extension (cyto- 

 pharynx) into the protoplasm. At the bottom 

 the cuticle is interrupted so that water and pro- 

 toplasm are in contact. By the action of the 

 cilia food particles are taken into the cyto- 

 pharynx and pressed into the protoplasm, 

 forming a small enlargement which finally sinks fig. i44.- 



■" t-aurlatu 



into the substance as a food vacuole (na), 

 which by the streaming of the j^rotoplasm is 

 carried about in the body. The digestible por- 

 tions are absorbed, and those not capable of 

 digestion are cast out of the body at a fixed 

 Ijoint (cytoiiyge) usually not recognizable at other times (fig. 151). 

 Contractile vacuoles (cv) are hicking only in parasites and 

 marine sjiecies. They are constant in number and position, and 

 frequently have aft'erent 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 placed vertically to the surface in the cortical 

 layer, which under the influence of reagents (chromic acid is best) 

 elongate into threads penetrating the cuticula. These have been 

 compared by some to the nettle cells of caslenterates, and have 

 been ascribed defensive functions; others regard them as tactile 

 structures. They have no connexion with the cilia. Nettle 

 bodies are extremely rare. Muscle fibres are more common ; they 

 lie between ectosarc and cuticle, and cause quick convulsive motions 

 of the animal. 



PaiYiniirciiim 

 <i (half (lia- 

 {^rammatic). <r, con- 

 tractile vacuole in 

 systole, cv', in dias- 

 tole; k, nucleus; un, 

 food vacuole, xa', in 

 formation; nk, micro- 

 nucleus; f, tricho- 

 cysts, at t' pz'otrTuled. 



