INFUSORIA. 



[ 417 ] 



INFUSORIA. 



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 cavi- 

 ties. By the older observers, they were 

 regarded as internal cavities into which 

 water was admitted with any particles acci- 

 dentally suspended in it, forming a means 

 of bringing a greater extent of surface of 

 the substance of the animalcule into con- 

 tact with the water, and thus aiding in 

 respiration. 



Ehrenberg regarded them as dilated caeca, 

 or portions of an alimentary canal (PI. 31. 

 tig. 38 ) j whilst Dujardin considered them 

 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 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. Sometimes a distinct oesophagus 

 lined with cilia leads to the internal sub- 

 stance, or to the sacculi. The course which 

 the particles (apparently in 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 and, more recently, Lachmann 

 admit either the existence of a distinct ex- 

 cretory orifice, or evacuation by the mouth, 

 other authors assert that these particles 

 may be evacuated at any part of the surface 



of the body. According to the later obser- 

 vations of Lachmann, the cavity of the 

 body of the Infusoria represents a large 

 digestive cavity, as in Hydra, the contents 

 constituting chyme, and there is a distinct 

 mouth and anus. 



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 par- 

 ticles drawn in by the cilia are urged at 

 random through the s,ubstance of the body. 

 The fact that distinct walls cannot be de- 

 tected, is of no great weight in opposition 

 to the former view, because the radiate 

 contractile vesicles of Paramecium exhibit 

 no walls, and are quite invisible when con- 

 tracted ; and the excretory vessels of Di- 

 stoma, although having distinct walls, are 

 seen to contract and then to vanish com- 

 pletely (Van Beneden). 



It may be easily ascertained by experi- 

 ment that some Infusoria will imbibe bi- 

 sulphuret 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 instance, greedily devours the 

 substance of crushed Entomostraca and 

 their ova, becoming greatly deformed in the 

 operation. 



The vacuoles or digestive cavities are 

 frequently very distinct when the ani- 

 malcules are dead, and especially when 

 dried. If the animalcules be fed with co- 

 louring-matter, on drying them, the vacuoles 

 thus rendered distinct will be found to con- 

 tain the pigment, which is in favour of 

 Dujardin's view. 



Surrounding the mouth in some Infu- 

 soria, as Nassula, Prorodon, Chilodon, and 

 Chlamidodon, is a horny cylinder of rod-like 

 bodies called teeth (PL 30. figs. 27 a, b, 29 ; 

 PL 31. figs. 40, 45, 72) : they do not appear 

 to exert any triturating power; and their 

 true signification is unknown. In some In- 

 fusoria an oesophagus is also present, as in 

 Vorticella, Carchesium, Epistyhs, Oxytricha, 

 &c., consisting of a mostly funnel-shaped 

 tube, often lined with cilia. 



A coloured gastric juice has been de- 

 scribed by Ehrenberg as existing in the 

 gastric cavities. The colour, however, has 

 been accounted for by Siebold as produced 

 by refraction and the presence of aggrega- 

 tions of pigment-granules, mistaken for gas- 

 tric cavities. This explanation we believe 



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