2 MOLLUSCA. 



ent points of the body, the chief of which, termed the brain, is 

 situated transversely on the oesophagus, and envelopes it with a ner- 

 vous collar. Their organs of motion and of the sensations have not 

 the same uniformity as to number and position, as in the Vertebrata, 

 and the irregularity is still more striking in the viscera, particularly 

 as respects the position of the heart and respiratory organs, and even 

 as regards the structure of the latter ; for some of them respire 

 elastic air, and others salt or fresh water. Their external organs, 

 however, and those of locomotion, are generally arranged symme- 

 trically on the two sides of an axis. 



The circulation of the Mollusca is always double ; that is, their 

 pulmonary circulation describes a distinct and perfect circle. This 

 function is also always aided by at least one fleshy ventricle, situated 

 between the veins of the lungs and the arteries of the body, and not 

 as in fishes between the veins of the body and the arteries of the lungs. 

 It is then an aortic ventricle. The family of Cephalopoda alone are 

 provided besides with a pulmonary ventricle, which is even divided 

 into two. The aortic ventricle is also divided in some genera, as in 

 Area and Linyula; at others, as in other bivalves, its auricle only is 

 divided. 



When there is more than one ventricle they are not joined in a 

 single mass, as in the warm-blooded animals, but are frequently 

 placed at a considerable distance from each other, and in this case the 

 animal may be said to have several hearts. 



The blood of the Mollusca is white or bluish, and it appears to con- 

 tain a smaller proportionate quantity of fibrine than that of the 

 Vertebrata. There are reasons for believing that their viens fulfil 

 the functions of absorbent vessels. 



Their muscles are attached to various points of their skin, forming 

 tissues there, which are more or less complex and dense. Their 

 motions consist of various contractions varying in their direction, 

 which produce inflexions and prolongations together with relaxations 



tion of the third volume for a long time after the appearance of the fourth ; among 

 the most prominent of which were the number of changes in the genera, and in the 

 distribution of species, he was compelled to make by recent discoveries. He also 

 acknowledges his obligations to the works of the late lamanted M. de Lamarck, and 

 those of MM.de Blainville, Savigny, Ferussac, Des Heyes, D'Orbigny, Rudolphi, 

 Bremser, Otto, Leuckart, Chamisso, Eisenhardt, Rang, Sowerby, Charles Desmou- 

 lins, Quoy and Gaymard, Delle Chiaje, Defrance, Deslonchamp, Audouin, Milne 

 Edwards, Duge"s, Moquin Tandon, Morren, Ranzani, and other savans whom 

 he names in different places. He concludes by regretting that he had not 

 received in time certain very recent works, which would have supplied him with 

 valuable materials, particularly the Syst. Acaleph., Berlin, 1829, 4to, of M. Esch- 

 holtz, and the article Zoophytes of the Diet, des Sc. Nat., of M. de Blainville, which 

 was not then published. ENG. Eo. 



