116 GENERAL STRUCTURE OF MOLLUSCA.—CEPHALOPODS. 
two first lie wide apart, but are connected by an arched 
band that passes over the gullet, e. The organs of sense in 
the higher forms of Mollusca are more developed than those 
of motion. They serve to direct the animal to its food, and 
to warn it of danger ; but there seems an absence, in all save 
the highest species, of that ready and acute sensibility which 
is so remarkable in the preceding groups ; and the variety of 
impressions which they can receive appears to be but small. In 
no instance has a special organ of smell been certainly dis- 
covered ; the organ of hearing is always imperfect, and fre- 
quently absent altogether ; and the eyes are very often wanting. 
In the lower Mollusca there are no certain indications of the 
existence of any organs of special sense ; and there is probably 
but a limited amount of general sensibility. 
111. As the Articulata are divided into two subordinate 
groups, according to the presence or absence of articulated 
limbs or members, so may we arrange the Mollusca in two 
subdivisions, according to the presence or absence of a dis- 
tinct head, that is, a projecting part of the body, containing 
the mouth or entrance to the digestive cavity, and also bearing 
the organs of sense which guide the animal in the discovery 
and selection of its food. In the higher Mollusca, there is a 
distinct head, furnished with eyes, and sometimes with im- 
perfect ears ; but in the lower, the entrance to the digestive 
cavity or stomach is buried deep among other parts, and is 
guarded by no other organs of sense than the tentacula or 
sensitive lips. These are termed acephalous, or headless 
Mollusca: and among the lowest of them (§ 114), we meet 
with compésite fabrics, formed by the process of multiplica- 
tion by budding, which was formerly regarded as peculiar to 
Zoophytes.—The highest group of Mollusca, in regard to the 
approach of several parts of its structure to that of Verte- 
brated animals, is the class of CrpHaLopopa, or Cuttlefish 
izibe : which receives its name from the peculiar arrangement 
of the arms or feet around the mouth, which is the cha- 
racteristic of its members (fig. 57). The common Cuttle-fish 
and its allies are destitute of any external protection ; but 
they usually have a flat shel!, commonly known as the cuttle- 
fish bone, inclosed in a fold of the mantle, and lying along 
the back. In the Calamary, this is horny in its texture, and — 
is sufficiently flexible to offer no resistance to the action of : 
