Races of Paramecium. 543 



There thus remain belonging to the genus Paramecium the 

 species aurelia, caudatum, bursaria, and perhaps putrinum ; these 

 are the four species accepted by Schewiakoff in his monograph 

 of the holotrichous infusoria ('96). Further, the two marine 

 forms P. marina Kent and P. pyriforme Gour. et Roeser may pos- 

 sibly belong to the genus; we have already shown that they have 

 no resemblance to the races that form the subject of this paper. 



Paramecium barsaria, as is well known, is a short and broad 

 form, the breadth being half or more than half the length. Ac- 

 cessible figures are given by Ehrenberg ('38), Kent ('82), Butschli 

 ('89), Schewiakoff ('96). To show the proportions, an outline 

 of Schewiakoff's figure is copied in our fig. 24, a. This species 

 is said as a rule to have rounded granules in the inner layer of 

 the ectosarc, and these, together with the endosarc, usually con- 

 tain green zoochlorellae, giving the animal a green color. Tricho- 

 cysts are generally present. But the rounded granules,the zoo- 

 chlorellae and the trichocysts may be lacking in some individuals. 

 The anus is at the posterior end. 



Paramecium putrinum CI. and L. is the same as P. bursaria, 

 save that it lacks the granules, zoochlorellae and trichocysts, and 

 has the anus on the ventral surface near the posterior end instead 

 of at the posterior end. As all these characteristics save the 

 last likewise occur in P. bursaria, the position of the anus remains 

 the only distinguishing feature. This slight variation in a most 

 undefined character would hardly appear to furnish grounds 

 for a specific distinction, as these distinctions are commonly 

 made, and as a matter of fact we find that Claparede and Lach- 

 mann described this as a separate species on entirely different 

 grounds. They say that they would not have attempted to dis- 

 tinguish this as a separate species, save for the fact that its em- 

 bryos are very different from those of P. bursaria ('68, p. 266). 

 Now these supposed embryos, as is well known, were really par- 

 asitic Suctoria, so that they have nothing to do with the specific 

 characters of the parasitized animals; it was doubtless accident 

 that the two sets of infusoria observed by Claparede and Lach- 

 mann happened to have different parasites. It is owing to this 

 accident that the two species have been estabhshed. 



