Page:Natural History, Mollusca.djvu/227

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215
TUSK-SHELLS.

merged shells and stones, known as the family Serpuladæ; and this position was assigned to the tusk-shells by Cuvier, even in his last edition of the "Règne Animal"

The shell, in this family, is a tube more or less curved, wider at one extremity than at the other, and open at both ends. It is, in fact, a very lengthened cone, with an open apex, as in Fissurella, a genus with which it has been considered to have some alliance.

The animal is of the same form as the shell, and presents not only in this respect, but also in many details of its structure, peculiarities which distinguish it from all the rest of the Gasteropoda. According to Mr. Clark, of Exmouth, to whose elaborate and skilful dissections our knowledge of the anatomy of the Mollusca is so much indebted, the gills are two symmetrical organs, hanging from the sides of the animal, a little behind its middle. The heart is placed at the front of the gill-cavity; a peculiarity of position which is dependent on the curious fact that the water to be respired flows into the gills from the posterior aperture of the shell. The front orifice is occupied and stopped by the thick collar of the mantle, through the centre of which the tip of the foot protrudes.
DENTALIUM,
(natural size)
without the shell.
The mantle invests the body like a tube; but if this be slit down the back, a minute head is discovered near the middle of the body, furnished with horny jaws, and bearing on each side a large tuft of filaments, which are considered to be salivary glands. There are neither eyes nor tentacles.