176 PROTOZOA. 



slightly separated that the structure of the whole organism is hardly 

 perceptibly modified. While in other animals tissues arise fitted 

 according to their properties for the discharge of various functions, the 

 Protozoa possess only a body-substance more or less homogeneous in 

 nature and similar in functional capacity. 



Since we have learnt to recognise in the cells of an organism not 

 only the ultimate anatomical elements, but also the physiological units 

 of all vital processes, and are, in other words, convinced that the 

 single cell lives and represents an organism (elementary organism, 

 according to Brlicke), it cannot any longer seem strange that there are 

 plants and animals which consist only of a single cell, or at least of 

 something equivalent, and which nevertheless perform essentially the 

 same vital processes as higher organisms, built up, perhaps, of millions 

 of cells. 



An animal of this kind possesses neither muscles nor nerves, and 

 has yet the capacity of motion and of sensation; it receives and 

 digests food without an alimentary canal, secretes without glands, and 

 reproduces its kind without sexual organs. All the functions which 

 in the higher organisms are severally discharged by diverse and de- 

 finite groups of cells and cell-derivatives, are here perfectly discharged 

 by a single cell, which constitutes the body of the animal. 



This body is not, however, always of perfectly homogeneous char- 

 acter; for in spite of their general simplicity, the Protozoa often 

 exhibit a certain histological and physiological differentiation similar 

 to that which we observe in the individual cells of higher organisms. 



Not only is the outer surface of the living protoplasm often hardened 

 to form a cuticle, which permits of ingestion and excretion only endos- 

 matically, but the protoplasm itself exhibits a differentiation into a 

 firmer outer portion, more or less exclusively the seat of motion (and 

 sensation), and a more fluid inner portion, which discharges the func- 

 tions of digestion and absorption. Like the protoplasmic contents of 

 the muscle cells, the contractile outer sheath can even acquire a 

 fibrillar consistency, by which it becomes fitted for even more energetic 

 discharge of its functions ; and through the contractility of the outer 

 portion, vacuoles, which originate there, become established as pulsat- 

 ing vesicles. Here and there, too, in the body-substance, holes and 

 openings appear, which, breaking through the cuticle and ectosarc, 

 represent the mouth and anus, much in the same way as similar 

 arrangements occur in the cells of higher animals (e.g., the so-called 

 " goblet-cells") for like purposes. In many cases (among the Infusoria) 

 further differences may be based even on the varied form and fate of 

 the nucleus. 



Thus we see that even among the Protozoa numerous and striking 



