Page:Picturesque Dunedin.djvu/109

Rh The next cases contain the star-fish, sea-urchins, and kindred species, as well as a series of models representing the various stages in the development of a star-fish, from the primordial egg, with its circular bands of cilia, and large brush-like appendage at one end, up to the final stage where a portion of the larva becomes differentiated to form a five-rayed star-fish. In the last model, the young star-fish, attached to the now useless remaining portion of the larva, is shown just ready to float away.

In the sea-urchin, the spines which give it such a forbidding appearance are more for defence than offence. Being attached by a ball-and-socket joint, they are movable and may be readily stripped off, the sea-urchin then presenting to view a hard calcareous case, dotted with five double rows of holes, through which in life, the tube-feet project. The habitation of this animal is chiefly in crevices of rocks, the spines projecting on every side, prevent it from falling, and when the sea-urchin wishes to move forward, the tube-feet are thrust out between the spines and fastened to the rock, then by contracting they draw the shell along.

Star-fish and sea-urchins appear to be quite dissimilar in structure, but a little examination and a comparison with allied species will show that there exists between them a close connection. The ordinary star-fish has the grooves along which its tube-feet project entirely oh the under surface, but there are other star-fish, shown in the museum, in which the arms are very much shortened, and have a tendency to turn upwards, showing the grooves on a side view as well as underneath. This is carried one degree further in the sea-urchin, which by the way is roughly pentagonal, and therefore represents the five rays of the star. The grooves, represented by five distinct dotted lines, are continued across the upper surface so as nearly to meet at the top. The external structure of the two animals is also very similar.

The class "Worms" follows next, and includes many well-known varieties, and among the rest some very good illustrations of parasitism.

As a general rule, parasitic animals are degenerated types, and simple in structure. Many of them have no mouth, or alimentary organs of any description, but simply absorb through