2 INVERTEBRATA OF MASSACHUSETTS. 



had no previous acquaintance, by the style of their architecture, 

 instead of making our observations, directly, upon the people 

 themselves. 



The true way is to unite the study of the animal and its shell ; 

 and, while we base the more general classification upon the struc- 

 ture of the former, we may characterize the species by their less 

 destructible habitation, the shell. The science thus extended, 

 has received the appropriate name, MALACOLOGY. 



As our observations are to embrace only a limited district, in 

 which many whole families of Mollusca are yet undiscovered, and, 

 indeed, are not likely to be found, and as detailed descriptions of 

 all the animals would be alike tedious and unprofitable to those for 

 whose benefit this Commission was intended, I shall incline to the 

 old plan, and confine myself principally to the shells. And while 

 the arrangement will be such as is actually indicated by the struc- 

 ture of the animals, so far as it has been ascertained, no allusion 

 will be made to them, unless materially to assist in the recognition 

 of species, or when, from their economical importance, some 

 knowledge of them is desirable. 



The naked Mollusca, as I before observed, are not numerous, 

 at least, they are not so with us. They are found both on land 

 and in water, and seem to occupy analogous positions in the dif- 

 ferent elements. Those found on land resemble the animals of 

 the snail-shell, and are ordinarily supposed to be those animals, 

 taking a stroll from their domicils. They are commonly called 

 slugs ; but their counterparts of the sea cannot with propriety be 

 called sea-slugs, inasmuch as that appellation is given to a very 

 different group of animals. 



As the species of naked Mollusca which have as yet been found 

 with us are few, it may be as well to enumerate them here, separ- 

 ate from the Testacea, though their position in a natural arrange- 

 ment would be very different. 



They all belong to that division of mollusks, which, in progres- 

 sion, glide along upon a lengthened foot extending under the body 

 ( Gasteropoda) j and with which we are familiar in any of our 

 snails, or the common periwinkles of our sea-coast. 



Those which are terrestrial seem all of them to belong to the 

 genus LIMAX. They have an elongated, tapering form, the head 



