DIFFERENTIATION OF THE BODY-SUBSTANCE. 231 



power of absoi-ption through the outer membrane. It is extremely 

 improbable that this mode of nutrition occurs in any free-living form, 

 although it is often so alleged. Albuminoid solutions soon putrefy, 

 and never occur around living organisims in a state which would 

 ensure all the properties essential for the nutrition of an animal ; so that 

 if they feed by imbibition, it must be on inorganic substance ; in other 

 words, they must be referred to the vegetable kingdom. A free-living 

 animal requires the power of incorporating a firm (i.e., resisting) 

 organic substance, and therefore, when a cuticle and cortical layer are 

 present, it requires a mouth. 



The Cortical Layer is, as we have indicated, the special seat of the 

 animal functions. This is proved not only by direct observation, but 

 by the fact that in the larger forms a striation is sometimes seen 

 resulting from a layer of fibrils. This occurs, for example, in the stalk 

 of the familiar bell- animalcule ( Vorticella), and presents a distinctly 

 muscular appearance. The course of the fibrils always corresponds 

 with the direction in which the contraction takes place, and is pre- 

 dominantly longitudinal. 



To this cortical layer belong the vacuoles, which, in the Infusoria, 

 usually occur at distinct places, and which, by their more or less 

 frequently repeated rhythmic contractions, convey the impression of 

 hearts. The resemblance is further increased when sometimes special 

 lacuDEB are found in connection with the pulsating space, as vessels 

 with the heart. These are especially visible at the moment of systole, 

 when the fluid contained in them is in a state of tension, and their 

 observation is as easy as it is beautiful, e.g., in the common slipper- 

 animalcule (Faramcecium aurelia). After contraction the fluid often 

 collects in little isolated drops, which afterwards flow together into a 

 larger vacuole. This is a certain proof that the vacuoles have no in- 

 dependent surrounding membrane, but owe their contractility solely 

 to the surrounding cortical layer. Their contractility varies consider- 

 ably in different species, being sometimes rapid and frequent, and in 

 other cases slow and at long intervals. Even in the same individual 

 differences are observable at different times and states ; but this may 

 at least be said, that in general the liveliness of the pulsations is pro- 

 portionate to the general energy of the animal and the temperature of 

 the surrounding medium. 



Our decision as .to the functional import of this apparatus de- 

 pends very essentially on the fact that in several cases it has been 

 most distinctly proved that it communicates with the exterior by an 

 opening, sometimes temporary, sometimes permanent, and rarely by 

 the anus. ^ Thus the contractile vacuole of the Infusoria so closely 



• See especially Wrz&Sjgi^e^'t^M/BJ^feS^/?-®"^*-. Bd. v., p. 25, 1869. 



