INFUSORIA. 



[ 343 ] 



INFUSORIA. 



at some part of the surface of the body, 

 when the spots will vanish. Different views 

 have been entertained in regard to the nature 

 of these spots or cavities. By the older 

 observers, they were regarded as internal 

 cavities, into which water was admitted with 

 any particles accidentally suspended in it, 

 forming a means of bringing a greater extent 

 of surface of the substance of the animalcule 

 into contact with the water, and thus aiding 

 in respiration. 



Ehrenberg regards them as dilated caeca 

 or portions of a true alimentary canal 

 (PL 24 a) ; whilst Dujardin considers them 

 as vacuoles arising in the same manner as 

 those found in sarcode, from whatever source 

 derived; others have viewed them as cells 

 floating loosely within the body. Most 

 observers deny that they are portions of an 

 alimentary canal, and that such canal exists, 

 but seem inclined to adopt the opinion that 

 they are cavities irregularly formed in the 

 substance of the body by the introduction of 

 the foreign matters, which are urged through 

 it by its contractions, or moved onwards by 

 its circulation. They are certainly not 

 cells, otherwise they could not so readily 

 admit particles of colouring matter, &c., nor 

 could their contents become fused together, 

 as is sometimes seen to occur. They do not 

 appear to be simply vacuoles filled in the 

 ordinary manner by the surrounding liquid, 

 because the pigment is accumulated in them 

 in greater proportion than it exists in the 

 liquid. In many Infusoria, the particles are 

 admitted at a definite orifice, representing a 

 mouth; this is round or oval, sometimes 

 situated at the anterior end of the body, 

 sometimes more posteriorly, or even at the 

 commencement of the posterior third of the 

 body, and it is generally indicated by a 

 circle, fringe or some other definite arrange- 

 ment of the cilia, which bring the particles 

 towards it. The course which the particles 

 (apparently the gastric cavities) take is 

 usually irregular, but sometimes tolerably 

 definite, down one side of the body and up 

 the opposite. The manner in which the 

 undigested particles are evacuated is also an 

 unsettled question; for whilst Ehrenberg 

 admits either the existence of a distinct 

 excretory orifice, or evacuation by the mouth, 

 other authors assert that these particles may 

 be evacuated at any part of the surface of the 

 body. 



The question then must remain, whether 

 there is a distinct alimentary canal, the walls 

 of which are invisible on account of their 



extreme delicacy, or whether the particles 

 drawn in by the cilia are urged at random 

 through the substance of the body. The 

 fact that distinct walls cannot be detected, is 

 of no great weight in opposition to the for- 

 mer view, because the radiate contractile 

 vesicles of Paramecium exhibit no walls, and 

 are quite invisible when contracted ; and the 

 excretory vessels ofDistoma, although having 

 distinct walls, are seen to contract, and then 

 to vanish completely (Van Beneden). 



It may easily be ascertained by experiment 

 that some Infusoria will imbibe bisulphuret 

 of mercury as readily as indigo or other 

 matters, and thus would appear to be entirely 

 deprived of any selecting power governed by 

 a sense of taste ; but some kinds would seem 

 to have a sense of taste : Coleps, for in- 

 stance, greedily devours the substance of 

 crushed Entomostraca and their ova, be- 

 coming greatly deformed in the operation. 



The vacuoles or digestive cavities are fre- 

 quently very distinct when the animalcules 

 are dead, and especially when dried. If the 

 animalcules be fed with colouring matter, on 

 drying them, the vacuoles thus rendered di- 

 stinct will be found to contain the pigment, 

 which is in favour of Dujardin's view. 



Surrounding the mouth in some Infusoria, 

 as Nassula, Prorodon, Chilodon, and Chla- 

 midodon, is a horny cylinder of rod-like bo- 

 dies, called teeth (PI. 23. fig. 27 a, b; fig. 

 29; PL 24. figs. 40. 45. 72) ; they do not 

 appear to exert any triturating power, and 

 their true signification is unknown. In some 

 Infusoria a kind of oesophagus is also pre- 

 sent, as in Vorticella, Carchesium, Epistylis, 

 Oxytricha, &c., consisting of a mostly fun- 

 nel-shaped tube, often lined with cilia. 



A coloured gastric juice has been described 

 by Ehrenberg as existing in the gastric cavi- 

 ties. The colour has, however, been ac- 

 counted for by Siebold, as produced by re- 

 fraction, and the presence of aggregations of 

 pigment-granules mistaken for gastric cavi- 

 ties. This explanation we believe to be in- 

 admissible; and in some instances at least (PL 

 23. fig. 19), the reddish-violet colour is real, 

 and arises from the presence of solution of 

 the chlorophyll of Oscillatorice, which is often 

 different by reflected and transmitted light. 



Circulating system. On closely watch 

 ing almost any of the Infusoria, minute, 

 mostly rounded, clear spots are seen in the 

 substance of the body, disappearing and re- 

 appearing at pretty regular intervals. These 

 are of variable size, but about that of the 

 gastric cavities. The nature of their con- 



