MOLLUSCA. 
3 
of their different parts, by means of which they creep, swim, and 
seize upon various objects, just as the form of these parts may permit; 
but as the limbs are not supported by articulated and solid levers, 
they cannot perform very rapid advances in progression. 
The irritability of most of them is extremely great, and remains 
for a long time after they are divided. Their skin is naked, very 
sensible, and usually covered with a humour that oozes from its 
pores ; no particular organ of smell has ever been detected in them, 
although they enjoy that sense ; it may possibly reside in the entire 
skin, for it greatly resembles a pituitary membrane. All the Cejjhala, 
Brachiopoda, Cirrhopoda, and part of the Gasteropoda and Ptero- 
poda, are deprived of eyes ; the Cephalopoda on the contrary have 
them at least as complicated as those of the warm-blooded animals. 
They are the only ones in which the organ of hearing has been 
discovered, and whose brain is enclosed with a particular cartila- 
ginous box. 
Nearly all the Mollusca have a development of the skin which 
covers their body, and which bears more or less resemblance to a 
mantle’, it is often however narrowed into a simple disk, or is formed 
into a pipe, or hallowed into a sac, or lastly is extended and divided 
in the form of fins. 
The Naked Mollusca are those , in which the mantle is simply 
membranous or fleshy ; most frequently however one or several 
laminae, of a substance more or less hard, is formed in its 
thickness, deposited in layers, and increasing in extent as well 
as in thickness, because the recent layers always overlap the old 
ones. 
When this substance remains concealed in the thickness of the 
mantle, it is still customary to style the animals Naked Mollusca. 
Most generally, however, it becomes so much developed, that the 
contracted animal finds shelter beneath it ; it is then termed a 
shell, and the animal is said to be testaceous ; the epidermis which 
covers it is thin, and sometimes desiccated it is called drapmar- 
inia). 
The variety in the form, colour, surface, substance and brilliancy 
* Until my labours on the subject -u-ere made public, the Testacea constituted a 
particular order; but there ai-e so many insensible transitions from the Naked 
Mollusca to the Testacea, and their natural divisions form such groups with each 
other, that this distinction can no longer e.xist. Besides this, there are several of 
the Testacea which are not Mollusca. 
0:3'“ name is given to a woolly texture which covers the outside of 
several univalve shells. Eng. Ed. 
B 2 
