. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 252 COMPARATIVE ZOOLOGY. Class III.—Echinoidea. The Sea-urchin is encased in a thin hollow shell cov- ered with spines, and varying in shape from a sphere to a disk.'" The mouth is underneath, and contains a dental apparatus more complicated than that of any other creat- ure from the Sponge to Man. It leads to a digestive tube, which extends spirally to the summit of the body. The spines are for burrowing and locomotion, and are moved by small muscles, each being articu- lated by ball-and- socket j
. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 252 COMPARATIVE ZOOLOGY. Class III.—Echinoidea. The Sea-urchin is encased in a thin hollow shell cov- ered with spines, and varying in shape from a sphere to a disk.'" The mouth is underneath, and contains a dental apparatus more complicated than that of any other creat- ure from the Sponge to Man. It leads to a digestive tube, which extends spirally to the summit of the body. The spines are for burrowing and locomotion, and are moved by small muscles, each being articu- lated by ball-and- socket joint to a distinct tubercle. ^ When stripped of ^^ its spines, the shell '7^ (or "test") is seen ^^ to be formed of a multitude of pen- tagonal plates, fit- ted together like a mosaic.'" Five double rows of plates, passing from pole to pole, like the ribs of a melon, alternate wnth five other double rows. In one set, called the amhidacra^ the plates are perforated for the protrusion of tubular feet, or suckers, as in the Star-fish. So that altogether there are twenty series of plates-—ten ambulacral, and ten interambula- cral. The shell is not cast, but grows by the enlargement of each individual plate, and the addition of new ones around the mouth and the opposite pole. Every part of an Echinus, even sections of the spines, show the princi- ple of radiation. If the arms of a Star-fish were turned. Fig. 20!).—Undei' of a Sea-nichiu (Ecluniis cscu- lentim), showiug rows of suckers aiiioug the spiues, British Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Orton, James, 1830-1877. New York, Harper and brothers
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishe, booksubjectzoology
