INTRODUCTION. XV 
Their locomotion consists of various contractions, which 
produce the inflections, prolongments or slackenings of 
their various parts ; at least, when they walk, swim or seize 
on different objects, according as the forms of their parts 
permit them: but when the members are not sustained by 
the articulated and solid levers, their movements are not 
rapid. 
Their irritability is generally extreme and remains a long 
time after any parts are divided from their body. Their 
skin is naked, very sensitive, and is generally endued with 
a liquor more or less mucous, which issues from their 
pores. No olfactory organ has been observed in any of 
the classes, although they appear to possess this sense: it 
seems that all the skin, which much resembles a pituitary 
membrane, has this sense. 
The eyes are wanting in all the classes excepting the 
Cephalopoda, who possess very perfect eyes; and in some 
of the Gasteropoda the eyes are situated on the tentacula 
(tentacles). 
All the animals of this group have a development of the 
skin which covers the body, which resembles more or less 
a mantle; which in some is capable of being contracted 
into a simple disc; in others, jom and form one or two 
tubes, or form a hollow sac; or lastly form fins. 
These animals have every sort of mastication and deglu- 
tition ; their stomachs are sometimes simple, sometimes 
multiplied, often furnished with peculiar hard armour-like 
substances ; and their interstices vary much in proportion 
and length. Almost all of them have salivary glands ; all 
have a liver of considerable size ; they have no pancreas or 
mesentery, and many of them have secretions proper to 
themselves ; but none of them produce urine. 
They exhibit all the varieties of generation. Many im- 
pregnate themselves; the others are hermaphrodites, and 
have need of reciprocal copulation ; im many the sexes are 
