18 



LECTUK 



described as stomachs by Ehrenberg, who has thence proposed for 

 this class the name of Polygastrica. The most minute forms, as the 

 species called Monas crepusculus, Ehr. have been estimated at the 

 ^-_^i_^ of a line in diameter.* Of such Infusoria a single drop of 

 water may contain five hundred millions of individuals, — a number 

 equalling that of the whole human species now existing upon the 

 surface of the earth. But the varieties 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 Monad to the Loxodes or 

 Ampliileptus^ which are one fourth or one sixth 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 aug- 

 mented by artificial aids. 



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

 delicate, transparent, and more or less ciliated integument. Others 

 are protected by a secreted 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 Euplcea 

 Charon ; others have their flinty armour resembling a minute bivalve 



shell : in some, as the Na- 

 viciday it has the form of an 

 elongated case, or flattened 

 cylinder, open at both ex- 

 tremities (^^.6.): it is some- 



6 



^^^^MlsMiiioyiWijiiijW®^^ 



Navicula. 



times straight, sometimes bent like the Australian boomerang ; it 

 may present the form of a reticulated cone {fig. 7.), or a discoid 

 case \^fig. 8.) ; in short, the varieties of the silicious 



shells of the Infusoria 



surpass in number those 



of the calcareous shells 



of the Mollusca. But 



whatever their forni, 



the superficies of these 

 Dictyocha. dclicatc microscoplc 



objects is generally sculptured with a beautiful, v/ell defined, and 

 more or less complicated pattern, which makes it easy to recognise 

 the species, and distinguish them from one another. 



Most of these animated minims are locomotive and free ; a few, as tlie 

 Vorticellje, are attached to foreign bodies by a long and highly irritable 

 and contractile pedicle ; others, as the Gomjihoncma, are appended to 

 the extremities of the branches of a dichotomously divided stem. 



Actinocyclus. 



* A line is tlie twtU'ih oi an inch. 



