$§ 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 afew of the Astoma, and these 
are, Cryptomonas ovata” and Opalina planariarum. 
§ 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 the water so communicates with 
their liquid contents as to perform a respiratory function. In this re- 
pect Actinophrys 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 ahead 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 caleareous, like the shells of Mollusca, and is not affected by heat. 
The Diflugiae carry about with them an envelope of this kind, composed 
of grains of sand. 
2 Ehrenberg, loc. cit. p. 41, Taf. II. fig. xvii. 
1 Bhrenberg (Ibid. p. 303, Taf. XXXI. fig. v1.1) appears to have taken the protrusion of these cou 
tractile vesicles for that of a snout. 
