OF MICRO- ORGANISMS. 53 



it casts its trichocysts upon its victim from a distance. 



The importance of this instance induces us to stop 



here a moment. 



The Didinium (fig. 7), 

 as regards the general 

 shape of the body, may 

 be compared to a dimin- 

 utive cask, rounded off at 

 one of the ends and term- 

 inated at the opposite ex- 

 tremity by an almost level 

 surface from the mids.t of 

 which rises a conical pro- 

 jection quite strongly 



Fig. 7.D.idinium nasutum, enlarged marked. This projection 

 two hundred diameters. The figure rep- . 



resents a Didinium overpowering a Pa- IS an Organ of deglutition 

 ramtecium aurelia. The nettle-like fila- 

 ments discharged by the Didinium are (swallowing) : a lonSfitU- 

 seen on all sides of the Paramczcium; 



while the latter, already seized by the dinal StriatlOn IS noticed 

 tongue-shaped organ of the Didinium, is 



being gradually drawn towards the buc- here formed of minute 

 cal orifice (after Balbiani). 



solid rods, of extreme ten- 

 uity and independent of the sides. These organs 

 are the weapons used by the Didinium in attacking 

 the live prey which constitutes its sole nourishment. 

 Not only does it attack and devour animalcula 

 almost as large as itself, but frequently it even seizes 

 individuals of its own kind. In such cases it is always 

 Infusoria, and never the Rotatoria, although the latter 

 often abound in waters which the Didinium inhabits. 

 It appears, moreover, to have a marked predilection 

 for certain species; and so it happens that the huge 

 and inoffensive Pafamcecium aurelia is almost always 

 its choice by preference among the animalcula that 

 inhabit the same liquid.* 



* The Didinium, Balbiani tells us, never attacks the Parma-cium bursaria, 

 which is distinguishable from the P. aurelia by its green coloration. 



