46 GENEUAL HISTORY OF 



just beneath the surface, are supposed (by Siebold,) to communicate 

 with the surrounding liquid. 



That a process similar to that of respiraton is required, is evidenced 

 by the necessity of a supply of air to the existence of Infusoria, 

 small though that sujDply need be. The more lively animalcules 

 require a more complete aeration of the fluid they inhabit, than do 

 the plant-like Bacillaria. 



That the faculty of secretion exists, and in an active form, is 

 proved by the production of the more or less hardened Lorica ; 

 keeping even the Bacillaria out of sight, the hardened cases, as of 

 Vaginicola, Cothurnia, Arcella, &c., or the glutinous investment of 

 Ophrydina, &c., bespeak its activity. 



Section XY. — Of organs of Sensation in the Polygastrica. — The 

 existence of any special organs of sensation in this class is but 

 hypothetical, but no doubt can be entertained that the Polygastrica 

 have a general sense of contact or of touch. Ehrenberg, however, 

 assumes that the coloured specs, seen in many, have a visual function, 

 and he consequently gives them the name of eyes, or eye-specks. 

 Thus he says, "In forty-eight species, included under the families 

 Monadina, Cryi^tomonadina, Vohocina, Astasicea, Binolryina, Peri- 

 dinaea, and Kolpodea, eyes are observable, and the colour of the 

 pigment is red in aU cases, except one, f OphryoglenaJ in which it ia 

 almost black. In connection with the visual organs of Amhlyophis 

 and Euglena, nervous ganglia have been seen, which constitute the 

 only traces of the evidence of a nervous system." 



The subjects of this section are thus referred to by M. Dujardin. 

 " The sense of sight would partake more of the character of reality, 

 if the colour of a speck without appreciable organization, without a 

 constant form or a precise contour, sufficed to prove the existence of 

 an eye. But, for instance, in the Euglena, which are particularly 

 cited as characterized by such an organ, the red spot so regarded is 

 excessively variable, sometimes multiple, at other times made up of 

 irregularly aggregated granules." ^ 



"Analogy, too, is inadequate to the solution of the question ; for, 

 on descending the animal scries, to detennine the nature of the 

 coloured speck, we have to leap from the Daphnioe (members of the 

 Entomostraca,) with a moveable eye, repeating in its composition 



