Page:Sea and River-side Rambles in Victoria.djvu/34

15 and the one, with the aperture on the right, which distinguishes it from the former is an allied genus, Limnæa, or Pond-snail. All these are widely distributed throughout the world. Now then let us watch how industriously they are cleaning off the dark coating of Conferva, which has already commenced growing on the sides of our Aquarium, and with this lens see how they use their tongue; the upper lip with its mandible is raised, the lower lip which is horse-shoe shaped, expands, the tongue is protruded and applied to the surface for an instant, and then withdrawn; its teeth glitter like glass paper, and in Limnæa it is so flexible that frequently it will catch against projecting points, and be drawn out of shape slightly as it vibrates over the surface.

Both these last species crawl beneath the surface of the water with their shells downwards, the Limnæa depositing its eggs, some fifty or sixty in number, in small sacs, and Mr. Hogg, in his valuable work on the Microscope, (Edition 4, p. 423,) gives us some interesting information on their subsequent development, " When examined soon after they are deposited, the vesicles appear to be filled with a perfectly clear fluid; at the end of twenty-four hours a very minute yellow spot, the nucleus or germ, may be seen near the side of the cell-wall. In about forty-eight hours afterwards, this small germ has a smaller central spot, rather deeper in color, which is the nucleolus. On the fourth day the nucleus has changed its position, and is enlarged to double the size upon viewing it more closely a depression or fissure is seen,—this on the