TUSK-SHELLS. 215 
merged shells and stones, known as the family 
Serpulade ; and this position was assigned to the 
tusk-shells by Cuvier, even in his last edition of 
the “Régne 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 Fissu- 
rella, 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 distin- 
guish it from all the rest of the Glasteropoda. 
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 de-. 
pendent 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. 
The mantle invests the body ar ee 
like a tube; but if this be (vratural size.) 
slit down the back, a minute Ree ci 
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. 
Ov 
Ze : 
= SEEN a 
_—_——_—_————_ 
Para 
