Page:Sea and River-side Rambles in Victoria.djvu/35

16 eight day most distinctly divides the small mass into the shell and soft part of the future animal. It is then detached from the side of the cell, and moves with a rotatory motion around the cell interior; the direction of this motion is from the right to the left, and is always increased when the sunlight falls on it. The increase is gradual up to the sixteenth day, when the spiral axis can he made out, and it presents a striking difference to the soft parts in appearance. On the eighteenth day these changes are more distinctly visible, and the ova crowd down to the mouth of the ova-sac; by using a higher magnifying power, a minute black speck, the future eye, is seen protruded with the tentacles. Upon closely observing it, a fringe of cilia is noticed in motion near the edge of the shell. It is now apparent that the rotatory motion first observed must have been in a great measure due to this, and the current kept up in the fluid contents of the cell by the ciliary fringes. For days after the young animal has escaped from the egg, this ciliary motion is carried on, not alone by the fringe surrounding the mouth, but by cilia entirely surrounding the tentacles themselves, which whips up the supply of nourishment, and at the same time the proper aeration of the blood is effected. Whilst in the ova it probably is by this motion that the cell contents are converted into tissues and shell. From the twenty-sixth to the twenty-eight day, it appears actively engaged near the side of the cell, using all its force to break through the cell wall, which at length it succeeds in doing, leaving the shell in the ova sac, and immediately attaching itself to the side of the glass vase to