256 DESCRIPTIONS OF PREPARATIONS. 



body has been observed to arise by fission from the nucleus. The facts 

 stated lead to the opinion that the paranucleus is nothing more than a 

 small nucleus. Paramecium is therefore bi-nucleate. Paranuclei are com- 

 monly found in Infusoria ; but in some multinucleate forms, as e.g. Opalina, 

 the nuclei are not only numerous but alike in size and other characters. 



The medulla (B : M.) forms the central portion of the body. It is 

 digestive in function and receives food-particles through a pharynx or 

 tubular prolongation inwards of the cuticle (B : /.). A groove commencing 

 on the left side of the animal leads obliquely to the entrance of this 

 pharynx. At its base is the cell-mouth or cytostome (B : 0.), where the 

 medullary protoplasm is bare. The food-particles collect here in a minute 

 drop of water (B : w."). Water and food enter the medulla, where they 

 circulate together as food-vacuoles (B : v.\ in the direction shown by the 

 arrows in the figure. The water in these vacuoles is slowly absorbed, while 

 the nutritive portion of the food is removed by a process not understood. 

 In some instances an acid reaction has been observed in the inclosing water. 

 The particles that remain are faecal, and, according to Saville Kent, are 

 expelled midway between the mouth and posterior extremity of the body. 

 It is not clear that Paramecium possesses a cell-anus or cytopyge lined by 

 cuticle such as exists in some forms. The granules in the medulla are 

 albuminoid and fatty in nature. When the organism has been starved for 

 a time the medulla becomes nearly hyaline : when full-fed, on the contrary, 

 perfectly opaque with granules. 



Reproduction takes place by the transverse division or fission of the 

 organism into two new individuals. The process is repeated for some time, 

 the resulting organisms gradually diminishing in size. Conjugation then 

 takes place. Two individuals fuse temporarily by their oral regions : and 

 during fusion there is a division of both nucleus and paranucleus (supra), 

 and apparently an interchange of protoplasm. When the conjugating 

 individuals separate, ' rejuvenescence ' sets in, i. e. the nuclei and paranuclei 

 are reconstituted, and each individual regains the full dimensions of the 

 species. Multiplication by fission then re-commences as before. 



Amoeba Proteus or A. princeps, the Proteus animalcule (Fig. 13 C.), 

 agrees with Paramecium in being a unicellular organism : it differs from it 

 in the absence of a permanently fixed cortex, and in the character of its 

 organs of locomotion, which are non-vibratile lobes of the protoplasm known 

 as pseudopodia (C :/. and/'.). It is to be found in the upper layers of soft 

 ooze at the bottom of still clear lakes, ponds, and ditches, or creeping on 

 the under surface of the fronds of Duckweed and the floating leaves of 

 various aquatic plants. 



The body consists of a pale, jelly-like, finely granular protoplasm, 

 capable of both extension and contraction at the will of the animal. The 

 outer layer of protoplasm retains a clear aspect and firm nature and con- 



