2 SECOND GRAND DIVISION. 
aortic ventricle is also divided in some genera as in Arca and 
Lingula ; at other times, as in other bivalves, its auricle only 
is divided. When there is more than one ventricle they are 
not united in a single mass, as in warm-blooded animals, but 
often sufficiently remote the one from the other, and then one 
might say there are many hearts. 
The blood of the mollusca is white, or bluish, and it appears 
to contain a smaller proportionate quantity of fibrine than that 
of the vertebrata. ‘There are reasons for believing that their 
veins fulfil the functions of absorbent vessels. 
Their muscles are attached to various points of their skin, 
forming there tissues, which are more or less complex and 
dense. Their motions consist of contractions in different 
directions, which produce inflexions and prolongations, or 
relaxations, of their various parts, by means of which they 
_ creep, swim, and seize upon objects, just as the form of these 
parts may permit; but as the limbs are not supported by arti- 
culated and solid levers, they cannot proceed rapidly, or by 
leaps. 
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 been 
discovered in them, though they enjoy that sense; it may 
possibly reside in the entire skin, for it greatly resembles a 
pituitary membrane. All the acephala, brachiopoda, cirrho- 
poda, and part of the gasteropoda, and pteropoda, are desti- 
tute 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 within a particular 
cartilaginous box. 
Nearly all the mollusca have a development of the skin, 
which covers their body, and which bears more or less resem- 
