CLASS OF THE GASTEBOPODA. 



515 



careous covering, but lives as a parasite in the shell of another 

 mollusc. 



This class comprises the octopus (Fig. 185), the argonaut 

 (Fig. 535), the sepia, the calmar (Fig. 532), the calmaret 



Fig. 538. The Ammonite j an extinct mollusk. 



(Fig. 10), the nautilus (Fig. 536), &c. With these are arranged 

 the ammonite shells, which have some analogy with those of 

 the nautilus, but are found only in a fossil state. 



CLASS OF THE GASTEROPODA. 



606. The gasteropoda are molluscs which have a head, 

 and move by means of a fleshy disc or foot placed under the 

 belly (Fig. 539), or of a fin formed by the same part of the 

 body (Fig. 543) : this class, which has as its type the snail, 

 is very numerous, and is composed chiefly of animals lodged 



Fig. 539. Helmet shell (Cassis). 



in a single shell, having generally the form of a cone, or 

 rolled into a spiral ; some species are, on the contrary, en- 

 tirely naked, as the slug. The body is elongated, and ter- 

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