PARAMECIUM AURELIA AND AMOEBA PROTEUS. 255 



refractile power, while the medulla is fluent, soft, more or less granular, and 

 opaque. The cortex is marked in many Infusoria, but not evidently in 

 Paramecium, by alternate light and dark lines, e. g. in Stentor. The former 

 corresponds to lines in which the cortex is thicker, and hence contraction 

 of the body takes place in a direction coincident with their length. The 

 cortical substance is doubly refractile, and hence depolarises light. This 

 fact is especially evident wherever it is thickened. 



The cortex contains the trichocysts, contractile vacuoles, nucleus, and 

 paranucleus. The trichocysts (B : /.) are not present in all Infusoria. They 

 are numerous in Paramecium, and form a superficial layer beneath the 

 cuticle. They are minute sacs containing a simple spirally coiled filament 

 which is eversible (B : //!), and therefore they closely resemble the nemato- 

 cysts of many Coelenterata. The contractile vacuoles are two in number, 

 one in front of the other. Saville Kent states that under normal conditions 

 they are evenly spheroidal in outline : but that under slight pressure they 

 are formed in a manner normal to certain types and exemplified in Fig. 13, 

 i, 2, 3, 4. Minute pyriform drops make their appearance pointing towards 

 a common centre. They enlarge, and their inner ends approaching give 

 origin to a central drop which swells as the radiating spaces disappear and 

 bursts externally, discharging its contents completely and disappearing in its 

 turn. During the process of transverse fission two vacuoles make their 

 appearance in the anterior region of the body, forming the vacuoles of one 

 of the two new individuals, the other retaining the two previously existent 

 vacuoles. The vacuoles are constant in position : their function is to 

 discharge superfluous water, containing perhaps soluble excretory products. 

 It has been observed by Mr. A. G. Bourne that when the organism is fed on 

 food-particles stained with aniline blue soluble in water, the dye is rapidly 

 excreted by the vacuoles in a concentrated form. 



The nucleus or endoplast (B : n.\ and paranucleus or endoplastule 

 (B : '.), sometimes erroneously termed nucleolus, lie in a thickening of the 

 cortex. In some ' instances they lie in the medulla and circulate with its 

 movements. They were at one time regarded as ovum and testis respec- 

 tively. In fission and during conjugation they divide, in fission once, in 

 conjugation twice at least : and in the latter case the segments of the twice 

 divided nucleus are usually further broken up. During the process of 

 division nucleus and paranucleus alike have been observed to become 

 striated. After conjugation, a process which is only temporary in Parame- 

 cium, and in which protoplasm is certainly exchanged between the con- 

 jugating individuals, both structures are reconstituted. The new nucleus 

 is said to be formed by the fusion of two portions of the twice divided 

 paranucleus. Some of the fragments- of the nucleus are stated to be 

 expelled, while the remaining fragments of both structures disappear unless 

 a few of them fuse to form a new paranucleus. But in Stylonychia this 



