Fig. 139. Echinarachnius, seen from above, with the spines on part of the shell; a ambulacral zone, i interambulacral zone. |
Beside the Toxopneustes ([Fig. 131]) described above, we have another Sea-urchin very common along our shores. Among children who live near sandy beaches, they are well known as "sand-cakes" ([Fig. 139]), and indeed they are so flat and round, that, when dried and deprived of their bristles, they look not unlike a cake with a star-shaped figure on its surface. ([Fig. 139.]) When first taken from the water they are of a dark reddish brown color, and covered with small silky bristles. The disk is so flat, being but very slightly convex on the upper side, that one would certainly not associate it at first sight with the common spherical Sea-urchin or Sea-egg, as the Toxopneustes is sometimes called. But upon closer examination the delicate ambulacral tubes or suckers may be seen projecting from along the line of the ambulacra, as in the spherical Sea-urchin; and though these ambulacra become expanded near the summit into gill-like appendages, forming a sort of rosette in the centre of the disk, they are, nevertheless, the same organs, only somewhat more complicated. When such a disk is dried in the sun, and the bristles entirely removed, the lines of suture of the plates composing it, and corresponding exactly to those of the spherical Sea-urchin, may very readily be seen. (a and i, [Fig. 139].)
Fig. 140. Transverse section of Echinarachnius; o mouth, e e ambulacra, c m ambulacral ramifications, w w interambulacra. (Agassiz.)
This flat Sea-urchin or Echinarachnius, as it is called, belongs to a group of Sea-urchins known as Clypeastroids (shield-like Sea-urchins). In a section ([Fig. 140]) exposing the internal structure, one cannot but be reminded by its general aspect of an Aurelia. Could one solidify an Aurelia it would present much the same appearance; another evidence that all the Radiates are built on one plan, their differences being only so many modes of expressing the same structural idea. The teeth or jaws in this flat Sea-urchin are not so complicated as in the Toxopneustes, being simply flat pieces, arranged around the mouth (o, [Fig. 140]), without the apparatus of muscular bands by means of which the teeth are moved in the other genus. It is a curious fact, considered in relation to the general radiate structure of these animals, that the teeth, instead of moving up and down like the jaws in Vertebrates, or from right to left like those of Articulates, move concentrically, all converging towards the centre.