26 $$ 11, 12. 
pigment, for the blue, violet and green pigments, seen in the eyes of in 
sects and crustacea, show clearly that the red pigment is not essential to 
the eye.* 
INFUSORIA AND RHIZOPODA. 
CHAPTER V. 
DIGESTIVE APPARATUS, 
§ 11. 
The Infusoria are nourished, either by taking solid food into the interior 
of their body, or by absorbing by its entire surface nutritive fluids which 
occur in the media in which they live. 
This last mode is illustrated in the Astoma, which have no distinct oral 
aperture or digestive apparatus. By the ingenious experiment first per- 
formed by Gleichen,” of feeding these animals with colored“liquids, no trace 
of these organs could be found. 
Ehrenberg, who also had observed that they did not eat, regarded their 
internal vesicles as stomachal organs, which were in eonnection with the 
mouth by tubes. The correctness of this opinion, however, has not been 
verified. Indeed, the genus Opalina® refutes it; here the species are quite 
large and visible to the naked eye, yet an oral aperture can be detected up- 
on no part of their body, and never do they admit into its interior colored 
particles. Solid substances found in them cannot be regarded as food. 
That fluids are here introduced by surface-imbibition is shown by Opaline 
ranarum ; this animal is found in bile in the rectum of frogs, and assumes 
a green color. When Opalina requiring only a certain quantity. of 
liquid are placed in water, they quickly absorb it, become greatly swollen, 
and shortly after die. In such cases, the absorbed liquid is seen as clear, 
vesicular globules under the surface, and these globules have been taken by 
Ehrenberg ag stomachal vesicles (VENTRICULI), and by Dujardin as 
VACUOLAE. 
§ 12. 
Those Infusoria which are nourished by solid food have a mouth at a cer- 
tain place, and an cesophagus traversing the parenchyma of the body. 
Through this last the food is received, and is finally dissolved in the semi-liquid 
parenchyma of the body, without passing through stomachal or intestinal cav- 
ities. In many cases there is at the end of the body opposite the mouth an 
anvs, through which the refuse material is expelled. But, when this is 
8“ Die Infusionsthienzhen,” p. 492. 
1 Auserlesene mikroskopische Entdeckungen, 
1777, p. 51; also, Abhandlung iiber die Saamen- 
und Infusionsthierchen, 1778, p. 140. 
* Some recent researches of T'huret (Ann. d. Sc. 
Nat. 3rd ser. XIV. 1850) on the reproductive germs 
of Algae prove that these bodies have red eye- 
like specks, resembling those seen in the Polygas- 
trica, but which disappear when the Zoospores at- 
tach th yes and germi pr ds. The 
2The genus Opalina was first established by 
Purkinje & Valentin. Many species are found 
in the rectum of frogs, and itis not rare to meet 
with them in the alimentary canal of Planarieae.t 
fact is a very interesting one in this connection. — 
Eb. 
+[§ 11, note 2.) According to Agassiz (Amer. 
Jour. Sc. XIII. 1852, p. 425), Opatina is only & 
larval form of Distoma. — Ep. 
