THE CONTRACTILE VESICLE OF PARAMECIUM. 169 



of complication which, perhaps, exceeds that of any other similar 

 organ. I have represented them in this figure in two of their 

 most striking conditions ; such as you would hardly suppose 

 could be exchanged the one for the other; and yet for hours 

 have I watched them, as clearly visible as they appear to your 

 eyes in the illustration, alternately assuming these two phases. 

 They both lie close to the thick wall of the body on the side 

 nearly opposite to that in which the mouth opens, but their 

 canals extend almost around to the latter point. When fully 

 expanded (cv) the main part of each vesicle is about half as 

 broad as the body, and there is little or no trace of the canals to 

 be seen ; but the moment contraction begins, they appear as fine 

 radiating streaks, and as the main portion lessens they gradu- 

 ally broaden and swell until the former is emptied and nearly 

 invisible, and they are extended over half the length of the body. 

 In this condition they might be compared to the arterial vessels 

 of the more elevated classes of animals, but they would at the 

 same time represent the veins, since they serve at the next mo- 

 ment to return the fluid to the main reservoir again, which is 

 effected in this very remarkable way. As the main vesicle (cv 1 ) 



9 



begins to expand, the canals, one after the other, and in nearly 

 regular sequence, suddenly open into it at their broad end (n) 

 with a sort of puff, or as if a valve had been jerked back, and, a 

 communication being thus established again (i) with the central 

 point, their contents are quickly poured out, and then in like suc- 

 cession they contract until nothing is to be seen of them, whilst 

 the vesicle expands to its fullest extent, as we see it (cv) in 

 the posterior half of this figure. The contents of these vesicles 

 is a perfectly clear fluid, without the least trace of granules, or 

 anything that might correspond to the blood corpuscles of those 

 animals which possess a true circulatory system. 



The reproductive organ is excessively faint, and very difficult 

 to detect, even with the best powers of the microscope, except at 

 certain seasons when the eggs are full grown. It consists of a 

 slender tube in which the eggs (n) are placed in a single line, 

 one after the other, at varying distances. It usually lies in the 



