Races of Paramecium. 



531 



24 



23 



Fig. 23. Dorsal {x) and lateral {y) views of a starved specimen of the race c 

 (aurelia), showing how under these conditions the posterior part of the body is 

 compressed dorso-ventrally. X 375. 



Fig. 24. Outlines of Paramecium bursaria (a) and P. putrinum (&), to show the 

 form. From the figures of Schewiakoff (1896). X 225. 



in the former case; the supposed difference between the two species 

 in this respect is probably to be accounted for in this way. 



(In preserved specimens, in many cases a few trichocysts 

 have been discharged from the posterior tip only, extending merely 

 some distance beyond the cilia; this is especially likely to happen 

 in races of caudatum. They then give the appearance of a very 

 pronounced bundle of long cilia in this region. Comparative 

 study will show however every gradation between such cases and 

 those in which the trichocysts have extended many times the 

 length of the cilia, so as to be at once recognizable as trichocj^sts.) 



Trichocysts. — It is said that in the species P. bursaria the tricho- 

 cysts are sometimes absent. It is worth noting therefore that 

 in all the races mentioned in this paper trichocysts are present. 

 Weak methylene blue readily causes their discharge ; and the dis- 

 charge usually occurs to a certain extent in killing fluids. 



Contractile vacuoles. — In Paramecium putrinum it is said that 

 there is usually but one contractile vacuole (Schewiakoff, '96, 



