146 



adapted to live in intestinal fluids accounts for the difficulty 

 whicli many have experienced in witnessing the contractions 

 of the contractile spaces in Opalines. It also accounts for the 

 oblong vessel surrounding the nucleus in some of the species 

 figured by Stein, as it most cevt^ixAy produces this appearance 

 in the Opalina which I have studied ; and it also explains the 

 nucleus-membrane as distinct from nucleus-content of which 

 Stein speaks. 



The long vessel of Opalina Planaria described by Schultze 

 may possibly be also due to such a separation of the inner 

 and outer layers of the organism by the imbibition of water, 

 since in my Opalinse the cavity so produced had most closely 

 the appearance of a long vessel, for which I mistook it at 

 iirst. In the Opalina of CHtellio I observed and drew a long 

 vessel, produced, I now believe, in this way. The im- 

 bibition of water, besides distending some of the vesicles, 

 may cause them to run together, and form much larger 

 lacunae than ever exist in the living animal, as in fig. 2, y. 



To avert the death and rapid post-mortem changes of the 

 Opalina), it is only necessary to avoid using water and let 

 the parasites, when extruded, remain in some of the fluids 

 from the worm. If arranged in this way they will be seen 

 as in fig. 1 ; and the rapid contractions of the series of 

 globular cavities arranged on one side of the creatures may 

 be watched when once the eye has got accustomed to their 

 movements, and very few more beautiful sights can be pre- 

 sented to the observer. 



As the Opalina rolls slowly over on to one side, the 

 attention may be fixed on one of the globular cavities, which 

 appears like a small jnnk bladder floating in the perfectly 

 colourless sarcode of the Infusorian. Suddenly as you watch 

 this bladder — so suddenly that you are almost startled — the 

 bladder is gone ; but almost immediately in its place a very 

 minute spot is seen, which slowly increases in size, and ulti- 

 mately proves to be the same cavity reappearing. 



Dr. Moxon having recently, in the ' Journal of Anatomy ' 

 (May, 1869), written on the subject of the contractile vesicle 

 of Infusoria, advancing arguments in favour of the view that 

 it opens externally, I may here point out certain aj)pear- 

 ances which are visible in Opalina, and which seem clearly 

 to accord with this view ; though, at the same time, I should 

 say that there can be no doubt about the opening of the 

 vesicle, after the paper of Dr. Zenke, noticed in this Journal 

 two years since. In fig. 2, in which the vesicles are dis- 

 tended by an excessive endosmose, they are seen to have a 

 lemon shape, more or less, the point being nearest the 



