145 



curiously inconsistent with the existence of the nucleus, the 

 contractile vesicles, and the ribbed ciliated cuticle of a typical 

 Infusorian. 



In PI. IX, fig. 1, is drawn a fair average specimen of the 

 Opalina infesting Nats serpentina. It is about the 1 -200th 

 of an inch long, flattened very considerably (so as to be 

 band-like), and of an oblong form. Hundreds are often 

 crowded together in the intestines of the Nais ; and so 

 tightly packed are they that frequently some specimens 

 grow into an irregular shape, as seen in fig. 8. They do not 

 vary much in size in the same worm ; but in Lumbriculus I 

 have seen what I believe to be the same species, as large 

 as one fiftieth of an inch, and with thirty contractile 

 vesicles. 



Many of the specimens brought out on to the field of the 

 microscope, by causing the worms to burst, are seen to be in 

 the act of transverse fission (fig. 7) ; and their increase in 

 size appears to be prevented by this removal of a portion of 

 the individual. Those that have been recently se2:)arated by 

 transverse fission have a pointed anterior extremity, which gra- 

 dually assumes a flat and blunted character as growth proceeds. 

 Usually when a Nais is placed on a glass slip, and squeezed 

 beneath the thin cover, so as to cause the Opalina to be 

 extruded, it is noticed that, though at first they move rapidly 

 about the field of the microscope with a regular and active 

 movement of their long cilia, yet slowly and surely these 

 movements become weaker and intermittent — the cilia alto- 

 gether ceasing and then resuming their action ; until at last 

 movement ceases altogether. When first extruded the 

 observer will have noticed the great transparency of the 

 Opalina, and will scarcely have been able to define the large 

 nucleus (fig. 1, «) indicated by a somewhat darker tint; but 

 he will have caught sight of the row of globular spaces 

 usually confined to one side of the body, and will probably 

 have watched in vain for their contraction. 



Now, Avhen the ciliary motion has ceased, it Avill be seen 

 that some of the globular sj^aces have acquired a great size 

 (fig. 2) ; whilst the nucleus has become very distinct, being 

 defined by a broad space running round it, and having the 

 same pink or purplish tint which is so characteristic of the 

 contractile vesicles of all Infusoria. The death of the Opalina 

 has been caused by its sudden introduction to fresh water — 

 the distension of its contractile spaces, and the formation of 

 the cavity between the nucleus and peripheral tissne being 

 equally caused by the rapid endosmose of water. It seems very 

 probable that this action of pure water upon an organism 



