STRUCTURE OF THE MOLLUSCA. 185 



elongated tube formed by the vertebrai, 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, I, 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 

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

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

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

 elongation and contraction of certain parts that they crawl 

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

 characterized by the precision observed in the higher ani- 

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

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

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

 and fiexile organs intended for locomotion. 



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

 a very complex vascular apparatus, composed of arteries and 

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

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

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

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

 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. 

 Sometimes there are seen at the base of the pulmonary 

 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- 

 ling the blood. 



The respiratory organs vary in their form and structure, 

 q3 



