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 composite 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 CEPHALOPODA, or Cuttle-fish 

 1f ibe : 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 shell, 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 



