Among the lower forms we find sea-hedgehogs, that is, "sea-urchins," and "sea-cucumbers." It is of these I would now briefly speak, as they are among the interesting things which the visitor to the sea-side will be sure to find, if he search faithfully for them. But first as to the sea-urchins (Figs. 26, 27). If one would study these strange forms, he must seek for them in the deeper pools left by the tide. Nor is a casual glance into the pool sufficient to reveal the prize
|Fig. 26.—Sea-Urchin (Toxopneustes drobachiensis, Agassiz).||Fig. 27.—Top-View of Sea-Urchin, Spines removed. Shows ambulacral and interambulacral plates.|
which he is in search of. The beginner may look into a clear pool where there are a hundred sea-urchins, and perhaps he will not see one until he has looked for some minutes; for sea-urchins not only resemble some of the sea-weeds in their color, but by means of their locomotive suckers they draw the sea-weeds closely about them, in many cases completely concealing themselves. If the collector reach down into the water the full length of his arm, and move his hand over the bottom among the sea-weeds, he will not be long in finding a sea-urchin. He will know when he touches one, as the sharp spines stand out on every side. Without moving from his position the collector will often secure a score or more of fine living specimens—some hardly exceeding a fraction of an inch in diameter, and others two inches or more; for he will find them of different ages, even if not of different species. Should he put them in a shallow pool while he goes on collecting, and then look for them again, he will at first think they have escaped into the sea or into some hole; for, true to their instincts, no sooner are they uncovered than they begin to conceal themselves again by drawing around them the sea-weeds by means of their long locomotive suckers. If we turn one over on his "back," that is, place the mouth upward, the urchin immediately begins to turn itself, and in a short time will regain its natural attitude, mouth downward.
The spines are very remarkable, not only in their appearance, but in their structure. A cross-section of one, under a microscope, reveals a structure so perfect and so beautiful that the richest mosaic is but rude masonry as compared with this natural mosaic.
Situated among the spines are curious three-pronged forceps, which have much puzzled the naturalists in days gone by; for it was doubtful