28G THE PROTOZOA 



be great variation, from long, slender to short, stumpy types, and 

 in some cases the posterior end is also greatly draAvn out and attenu- 

 ated. The principal nucleus or trophonucleus is usually situated 

 near the middle of the body. The kinetonucleus is almost invariably 

 behind the trophonucleus,* sometimes close behind it, but more 

 usually near the posterior extremity, separated from the tropho- 

 nucleus by about half the length of the body. 



The flagellurn arises from a centriole (blepharoplast) which is in 

 connection with the kinetonucleus. In the more primitive type of 

 arrangement the blepharoplast is lodged within the kinetonucleus 

 itself, and then the flagellum appears to arise from the kineto- 

 nucleus directly (Wenyon, 84). In most cases, however, the 

 blepharoplast is situated close beside, and usually in front of, the 

 kinetonucleus, connected with it by a delicate rhizoplast. When 

 the blepharoplast is distinct from the kinetonucleus, it is at present 

 an open question whether the kinetonucleus contains a centriole 

 of its own, in addition to the blepharoplast, or whether the blepharo- 

 plast represents a centriole which belongs to the kinetonucleus, but 

 has migrated to the exterior of this body. 



Passing from the blepharoplast to the surface of the body, the 

 fiagellum forms the free border of the undulating membrane, which 

 runs forward from the vicinity of the kinetonucleus to the extreme 

 .anterior end of the body as a fin-like ridge or fold of the periplast, 

 of variable width (c/. Fig. 126). The flagellum may in some cases 

 end with the undulating membrane at the anterior end of the body, 

 but more usually it is prolonged forward beyond this point, so that 

 a free portion of variable length is to be distinguished from the mar- 

 ginal portion contained in the undulating membrane. The sinuous 

 body, the undulating membrane, and the flagellum, are alike in a 

 state of incessant movement during life, and in larger forms con- 

 tractile myonemes are clearly visible in the periplast of the body 

 (Fig. 28, p. 58) ; in the more minute individuals the presence of such 

 elements must be inferred from their movements, but cannot always 

 be demonstrated optically. 



The movements of a trypanosome, speaking generally, are of two types : 

 travelling movements, when it progresses with the free flagellum forwards, 

 sometimes very fast, shooting across the field of the microscope in a straight 

 line (mouvement en fleche), sometimes, on the other hand, pushing its way 

 .slowly through the blood-corpuscles, with the flagellum directed either forwards 

 or backwards in movement ; and wriggling movements, when the animal 

 writhes incessantly in serpentine contortions with little or no displacement 



* The only known exceptions are furnished by certain forms of the recently- 

 described T. rhodesiense (vide Stephens and Fantham), and by some of the small 

 forms seen during the multiplication of T. lewisi (Fig. 127, L). It is needless to 

 point out that the statement made above applies to the typical trypanosome-forra 

 as found in the vertebrate blood, and not to the developmental forms through which 

 they pass in the invertebrate host (crithidial and other types). 



