POLYGASTRIA. 19 



serial animalcules, of a higher grade of organisation than the 

 more diffusely ciliated group {figs, 5. & 6.), which is the subject of the 



present lecture. 



The species of this group 

 are essentially nucleated 

 cells, with a superadded 

 organisation for locomotion, 

 digestion, and, in some spe- 

 cies, for circulation and ge- 

 neration. The cell-wall in- 

 closes a colourless and trans- 

 parent gelatinous plasma, 

 with numerous minute and 



1-3. Vibrio .ubtilis ; 4, 5. Vibrio ru^ula ; 6, 7. Closterium. ^^^^^ COlourcd COrpUSClcS. 



Particles of food wlien 

 taken in are seen to occupy subspherical and subequal spaces in the 

 same plasma, and the spaces so occupied being regarded by Ehrenberg 

 as stomachs, he has thence proposed for this class the name of Poly' 

 gastria, which may well be retained, although the idea of these 

 assimilative cavities having distinct walls and intercommunicating 

 canals be abandoned. 



The most minute forms, as the species called Monas crepusculus, 

 Ehr., have been estimated at the 2^^^^ ^ of a line in diameter.* Of 

 such Infusoria a single drop of water may contain five hundred mil- 

 lions of individuals, — a number equalling that of the whole human 

 species now existing upon the surface of the earth. But the varie- 

 ties in the size of these invisible animalcules are not less than that 

 which prevails in almost every other natural class of animals : from 

 the minutest ilTowttc? to the larger species oi Loxodes or Amphileptus, 

 which are one sixth or one fourth of a line in diameter, the difference 

 of size is greater than between a mouse and an elephant. Within 

 such narrow bounds might our ideas of the range of size in animals 

 be limited, if the sphere of our observation was not augmented by 

 artificial aids ! 



Many of the polygastric animalcules are naked, covered only by an 

 elastic, transparent integument, which, in some {Euglena^ Amoeba\ 

 is smooth ; in most is more or less ciliated. Others are protected by 

 a shell, which consists of pure, colourless, and transparent silex. 

 This shell may present the form of a simple shield, indicating by its 

 position the back of the animal, as in Eiiplcpci Charon ; others have 



* A line is the twelfth of an inch, 

 c 2 



