360 



NATURAL HISTORY. 



Fig. 10. A, PRORODON MARGA- 



RITIFER; B, PHARYNGEAL ROU- 

 FASCICLE OF P. NIVEUS C, 

 DITTO OF NASSXJLA. 



cv, contractile vesicle ; n, nucleus. 



One or more food vacuoles may exist, and as one is formed subsequently to the other, the oldest 



vacuole is the most deeply imbedded, and if the animalcule be fed with carmine, a number will be 



noticed forming a series in the endoplasm, and moving with. it. Much of the food thus received is 



digested, and the rest is evacuated in a definite direction, and sometimes through a special opening 



in the ectoplasm the anus. In many species, however, the faecal matters pass out at any point. 



When one of the Infusoria is lively and feeding, and is being examined under high powers of the 

 microscope, one or more spots, with a circular or radiating outline, will 

 suddenly appear near the ends of the body. Each begins in a point 

 of greater transparency than the body structure all around it, increases 

 rapidly in diameter, and often assumes a tinge of colour, retaining, how- 

 ever, its transparency. It is a light-transmitting space, with the slightly 

 denser structure of the inside of the animal around it. As the light 

 from the reflecting mirror of the microscope traverses the tissue of 

 the Infusorian at this now enlarged spot, it seems to be unsteady, and 

 this depends upon water passing into this really globular space, which, 

 seen \mder the microscope, presents the appearance of a circular area 

 (Fig. 11, A, cv). It is evident that water flows into this space, which is 

 situated really in the layer immediately over the soft endoplasm ; 

 there is no environing membrane to it. Suddenly the circle of light 



closes in on its centre, and the appearance of a light point amongst the darker surrounding 1 



matter suddenly ceases. The tissue closes in on the space, moving in on all sides, and this is done 



not passively but actively, for in some instances a tremor can be seen to occur over the whole 



animalcule at the time of the contraction of the space. Moreover, although the space enlarges slowly, 



it contracts very rapidly, as a rule. If the Infusorian be kept for some time under observation, the 



absence of food and fresh water will begin to diminish 



its energies, and it will especially influence the rapidity 



of the dilatation and subsequent contractions of this 



space, which is termed a contractile vesicle. The 



appearance of the light spot is not so frequent ; it 



commences languidly, and enlarges slowly, and finally 



contracts, or disappears less abruptly than in the 



instance of the vigorous animal. After a while, 



the appearance and disappearance of the spot or, 



in other words, the dilatation and contraction of the 



contractile vesicle become slower and irregular, and 



they cease with the death of the animal. 



More than one contractile vesicle may exist 



in the same species, and their position in the body, 



although generally well defined, is not invariably in 



the same spot. Usually, the vesicles are nearer the 



ends of the body than the central part, and when Fi g- n A > niAGRAM OF BTICELIA NEBULIFERA : 



J . B-D, PHASES OF LONGITUDINAL FISSION J E-F, 



they are fully dilated they occupy not only a portion PHASES OF ATTACHMENT, AND G-H, OF INCORPORATION 

 of the body hitherto filled with endoplasm, but come 



. ,, , , ,. T ., . 



close under the outer and denser tissue. In the in- 



d, disc: p, peristome; 7, gullet: c, contra 



stances where the contractile vesicle presents the ^parUcTes? 31 ' Thearrow9d ' 

 appearance, under the microscope, of a circular space, 



no movement can be seen, in the vast majority of observations, to extend from it into the 

 endoplasm during the active contraction or dilatation. The water contained in the vesicle must 

 go somewhere, and must be derived either from within the body or from without, or perhaps 

 from both directions. Occasionally, however, a very indistinct movement can be seen radiating, as it 

 were, amongst the granular, or almost homogeneous protoplasm of the animal, subsequent to an active 

 contraction of the space. No visible movement accompanies the infilling. There are many Infusoria, 

 such as the species of Paramecium, in which the contractile vesicle, when fully expanded, is not limited 



OF FHEE EMBRYO OF V. MARINA (After Greeffe) ; 



I, ENCYSTED STAGE OF V. MICROSTOMA. 



, gullet: c, contractile vesicle: n. nucleus : 



