62 ORGANIZATION OF THE INFUSORIA. 



or so altered by deglutition as to present a homogeneous, horn-like con- 

 sistence ; each fine longtitudinal stria recognizable in the branching stalk 

 under normal conditions, indicating, again, the integral portion contributed 

 towards the formation of the whole by the separate members of the 

 terminal uvella-like colony. In certain other recently discovered forms, 

 as, for example, Stein's new genera Rhipidodendron and Cladomonas, a more 

 or less extensively branching tubular structure or " zooaulon " is built up, 

 into the composition of which, in the first-named genus more especially, 

 many hundred tubules not unfrequently enter. Each of these separate 

 tubules represents in either case the excreted product of the single animal- 

 cule or zooid which is found occupying its distal extremity, and which is 

 undoubtedly formed in a manner corresponding closely with that of the 

 pedicle of Anthophysa, though in this instance the excretion of digested 

 particles and mucus takes place throughout the greater portion of the area 

 of the periphery, instead of being limited only to the posterior region of 

 the body. 



Encystment. 



The phenomenon of encystment or cyst-development, briefly referred 

 to in a preceding page, represents so important a factor in the life-history 

 of the infusorial animalcules as to demand separate and extended notice. 

 As there intimated, this encysting process is found to exhibit many dis- 

 tinct and independent phases. In the first, and most general of these, 

 encystment may be defined as a mere conservative act resorted to by 

 any independent infusorium in the presence of conditions unfavourable 

 to its welfare, such as the change of temperature, or the drying up of the 

 surrounding water, or other inhabited medium. In this simply " protective 

 encystment," as instanced by Paramecium, Trachelius, and other free-swim- 

 ming types, the animalcule loses its accustomed activity, and settling down 

 in some chosen spot becomes, after a short duration of purely rotatory 

 movements, perfectly quiescent. The cilia now gradually disappear, 

 the animalcule at the same time contracts into a more or less perfect 

 spheroidal form, and exudes from its entire periphery a soft, mucilaginous 

 envelope, at first visible only as a delicate bounding line, but which harden- 

 ing by degrees assumes the nature of a transparent, membranous or shelly 

 capsule. Although in most instances these protective cysts present a simple 

 spheroidal contour and smooth homogeneous surface, several prominent 

 deviations are to be found. Thus in some instances this cyst or capsule 

 is double-walled, the exterior wall being, as shown by Auerbach in the case 

 of Oxytridia pellionella, soft and granular, and the inner one membranous 

 and elastic. In Stentor caruleus, again, the cyst (PL XXIX. Fig. 15) 

 is flask-shaped, and provided at its upper extremity with a close-fitting 

 operculum-like lid. In Euplotes charon, as shown by Stein, this same struc- 

 ture presents numerous longitudinally disposed, serrated, crest-like elevations, 

 which communicate to the capsule a somewhat melon-shaped outline. A 



