STRUCTURE OF THE MOLLUSCA. 185 



elongated tube formed by the vertebrae, or bones of the 

 neck, back, loins, and sacrum. The organs of sense are 

 always less perfect than in the Vertebrata, but differ ex- 

 tremely in the various groups. Some Mollusca appear to 

 have only the senses of touch and taste ; but in a great 

 number there are eyes, fig. 1, e; fig. 3, d; of which the 

 structure varies; in a few only there are organs of hearing; 

 but in none of them has a particular organ for smell been 

 proved to exist, although many of them appear to have the 

 faculty of smelling, which perhaps is exercised by the whole 

 surface of the body. 



There being neither an internal jointed skeleton, as in 

 the Vertebrata, nor an external skeleton, composed of hard 

 I jointed pieces or rings, as in the Articulata, the muscles are 

 I attached to different points of the skin, and act only upon 

 I the parts on which they are inserted, so that it is only by the 

 j elongation and contraction of certain parts that they crawl 

 i or swim, and their movements are generally slow, and not 

 I characterized by the precision observed in the higher ani- 

 mals, or in insects. They never have feet arranged in series 

 I on each side of the body, as in the Vertebrata and Articu- 

 j lata; and it is only in a few of them that there are elongated 

 j and flexile organs intended for locomotion, 

 i Their blood is white, bluish, or limpid, and circulates in j 

 I a very complex vascular apparatus, composed of arteries and 

 i veins. The circulation is always double; that is, the blood 

 j passes through two sets of capillary vessels, one set distri- 

 I buted in all parts of the body, the other belonging to the 

 I respiratory apparatus. The heart, fig. 9, /, formed of a 

 j ventricle and one or two auricles, receives the blood which 

 [ comes from the respiratory system, and impels it into the 

 arteries which distribute it to the various parts of the body. 

 I Sometimes there are seen at the base of the pulmonary 

 I arteries bags which receive the venous blood, and which 

 [ some have erroneously considered as so many hearts, they 

 being only receptacles, and not having the power of impel- 

 I ling the blood. 



The respiratory organs vary in their form and structure, 

 i q3 



