14 INTRODUCTORY. 



protruding a clavate proboscis ; feelers numerous, ciliate, capil- 

 lary, seated round the mouth. 



6. Nereis. — Body long, creeping, with numerous lateral 

 peduncles, or feet, on each side ; feelers simple, rarely wanting ; 

 eyes two or four, rarely none. 



7. AsciDiA. — Body fixed, roundish, and apparently issuing 

 from a sheath ; apertures two, generally placed near the upper 

 end, one beneath the other. 



8. Tethys. — Body detached, rather oblong, fleshy, without 

 peduncles ; mouth vvilh a terminal cylindrical proboscis, under 

 an expanded membrane or lip ; apertures two, on the left side of 

 the neck. 



9. Triton. — Body oblong ; mouth with an involute spiral 

 proboscis ; tentacula, or arms, twelve, six on each side, divided 

 nearly to the base, the hind-ones cheliferous. 



10. Sepia. — Body lieshy, receiving the breast in a sheath, with 

 a tubular aperture at its base ; arms eight, beset with numerous 

 warts or suckers, and in most species two pedunculated tenta. 

 cula ; head short ; eyes large ; mouth resembling a Parrot's 

 beak. 



It is upon the exclusive shape of the shell, and not the animal 

 inhabitant, that the Linnean arrangement of Conchology is 

 formed. 



In early periods, naturalists were in much doubt whether to 

 found their Conchological arrangements on the animal, or the 

 shells ; and at last it was determined, that it should be the latter ; 

 because few of the marine shells that were then found had the 

 animal in them, prior to the time when dredging was resorted to. 

 The greater part of shells in many collections at the present day, 

 have been found upon the beach, divested of their animal, having 

 been cast ashore by the agitation of the sea in storms ; and the 

 inhabitant decayed before they were picked up. 



Many shells are very diflFerent in their young and adult state 

 both in form and colour ; the student ought, therefore, to make 

 himself well acquainted with the generic characters, to enable 

 him to discriminate the difference. But, indeed, this is often a 

 difficult matter, even to those who have studied the science for 

 many years. 



In almost all the Cypreas, there is considerable difficulty in dis- 

 tinguishing them from the Volutae and Bullae, in their young statej 



