<§>«§. 18, 19. INFUSORIA AND RHIZOPODA. 31 



though it were a drop of oil. The observation of these phenomena would 

 make it doubtful whether or not these cavities are true vesicles or vessels. 

 These cavities have been met with in only a few of the Astoma, and these 

 are, Cryptomorms ovata ^^' and Opalina plariariarum. 



§ 18. 



The Infusoria appear to respire solely by the skin. In those species 

 whose bodies are covered with vibratile cilia this function is promoted by 

 the vortical action of the water caused by these organs. In others, the 

 contractile cavities just described are situated immediately under the skin^ 

 and the opinion may be entertained that tlie water so communicates with 

 their liquid contents as to perform a respiratory function. In this re- 

 pect Acthiophrys sol is quite remarkable, for its contractile cavities are 

 so superficial that when filled they raise the skin in the form of aqueous 

 vesicles,*^' which, however, are so elastic as entirely to disappear in the 

 parenchyma. Here it is plain that a mutual relation between the external 

 water and the contents of these cavities might easily take place. 



CHAPTER VIII. 



ORGANS OF SECRETION. 



§ 19. 



No special organ of secretion has been found in the Protozoa ; their 

 skin, however, has a power of secreting various materials, which in some 

 species harden and form a carapace, or a head of a particular shape ; while 

 in others it serves to glue together foreign particles, forming a case, in which 

 the animal retreats. 



Among those having a carapace, may be mentioned Vaginicola, Cothur- 

 nia, and Arcella. This more or less hard envelope does not resist fire, 

 and is probably of a corneous nature. In the Rhizopoda, however, it is 

 usually calcareous, like the shells of Mollusca, and is not affected by heat. 

 The Difflugiae carry about with them an envelope of this kind, composed 

 of grains of sand. 



2 Ehrenberg, loo. cit. p. 41, Taf. II. fig. xvii. 



1 Ehrenberg (Ibul. p. 303, Taf. XXXI. flg. vi. 1 ) appears to have taken the protrusion of these coB» 

 tractile vesicles for that of a snout. 



