48 IKTEODUCTIOX TO CONCHOLOGY. 



a kind of oblong mass, capable of fitting into the aperture 

 of the well-known shell. The outer portion presents a 

 perfectly developed head, furnished with a pair of strong 

 horny mandibles, and a number of sheathed tentacles, a 

 pair of large eyes also, and organs for hearing and smelling ; 

 and over all these extends a capacious leathery hood, with a 

 concavity fitting to the convex form of the shell, and having 

 apparently a hinge-like movement, analogous to the lid of a 

 box, by means of which the tentacles and delicate portions 

 of the head may be readily covered over. The lower portion 

 of the animal contains such viscera as are essential to her 

 welfare; she has, moreover, a central tubular membrane, 

 which, passing the siphon apertures of the different com- 

 partments, extends completely through the convolutions of 

 the shells from one chamber to another, until it is fastened 

 to the inner wall of the first-formed chamber. And, as if 

 further to strengthen this all-important ligament, around it 

 is a thin layer of horny matter called the belt ; and the mem- 

 brane itself possesses a degree of elasticity which readily 

 enables it to be extended as necessity requires. The creature, 

 therefore, can let herseK down, and whirl round and round, 

 as it were upon her axis, by the limited extension of this 

 membranous pulley ; which operation, however, ceases when 



