PROTOZOA 17 



nucleus by a thread or threads known as rhizoplasts (Fig. 47 A). 

 Sometimes it lies against the nucleus. Rhizoplasts may connect it 

 to other structures, notably in many parasitic flagellates to a body of 

 unknown function called the parabasal body. The " kinetonucleus " 

 of Trypanosoma (Fig. 47 E) is a body of this class, which possibly in- 

 cludes structures of more than one kind. Sometimes, as in Trypano- 

 soma^ a flagellum runs for some distance parallel with the surface of 

 the body and is connected to it by a film of protoplasm known as an 

 undulating membrane^ which must be distinguished from the structures 

 of the same name which are formed by the fusion of cilia. When there 

 are two flagella, it often happens that one is trailed behind the body 

 and the other directed forwards (Figs. 47 D, 53 C, 70 B). Flagella are 

 often used for anchoring, and sometimes appear to have a sensory 

 function. Cilia are smaller and more numerous lashes which by a 

 rowing action repeated by one after another of them in "metachronal 

 rhythm " (Fig. 13) cause movements of the animal or of the water near 

 it. Like flagella they have each an internal filament, a basal granule, 

 and a rhizoplast, which, however, does not connect with the nucleus. 

 Often cilia are united into compound organs, which may be conical 

 am, paddle-like membranellae (Fig. 14), or undulating membranes 

 (Fig. 90). Many protozoa which possess a definite body form are able 

 temporarily to alter it by contractions of the protoplasm stretching 

 the pellicle (metaboly), and in various cases this contractility is 

 localized in fibrils, known as myonemes^ situated in the ectoplasm. 



Systems of fibres which ramify from a central mass known as the 

 "motorium" and have been thought to be of the nature of a nervous 

 system have been described in various ciliates ; in some of these cutting 

 the fibres is said to destroy the co-ordination between different sets 

 of ciliary organs. It is possible also that the rhizoplast system of 

 flagellates may have a conducting function. Setise organs are possessed 

 by various protozoa in the form of specialized flagella and cilia in 

 which the tactile sense is highly developed, and by many of the plant- 

 like flagellates as pigment spots (eye-spots), which may be provided 

 with a lens. A chemical sense seems to be indicated by the fact that 

 food is often recognized at a distance, and also probably in some of the 

 cases of discrimination in ingestion (p. 19). 



Internal skeletal structures are found in many members of the 

 phylum. They may be part of the living protoplasm, as the axial fibres 

 of axopodia and the axostyles which lie in the midst of the body of 

 various mastigophora (Trichomofias, Fig. 50; etc.) and probably the 

 central capsules of the Radiolaria, or of dead inorganic matter, as the 

 skeletons of the Radiolaria (Fig. 69). 



The Protozoa present every type of nutrition exhibited by organ- 

 isms, except that of the "prototrophic" bacteria, which perform 



