MOLLUSCA. 
2 
ent points of the body, the chief of which, termed the brain, is 
sitnated transversely on the oesophagus, and envelopes it with a ner- 
vous collar. Their organs 9f motion and of the sensations have not 
the same uniformity as to number and position, as in the Vertehrata, 
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 lespiie 
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 ia distinct and perfect circle. This 
function is also always aided by at least one fleshy ventricle, situated 
betAveen the veins of the lungs and the arteries of the body, and not 
as in fishes between the veins of the body and the ai’teries of the lungs. 
It is then an aortic ventricle. The family of Cephalopoda alone are 
provided besides Avith a pulmonary ventricle, Avhich is even divided 
into tAVO. The aortic A'entricle is also divided in some genera, as in 
Area and Lingula; at uthers, as in other bivalATS, its auricle only is 
divided. 
When there is more than one A’^entricle they are not joined in a 
single mass, as in the AA’arui-blooded animals, but are frequently 
placed at a considerable distance from each other, and in this case the 
animal may be said to haA^e seA'eral hearts. 
The blood of the Mollusca is Avhite or bluish, and it appears to con- 
tain a smaller proportionate qiiantity of fibrine than that of the 
Vertehrata. 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, Avhich are more or less complex and dense. Their 
motions consist of various contractions A’arying in their direction, 
which produce inflexions and prolongations together Avith 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 ehanges in the genera, and in the 
distribntion of species, he was compelled to make by recent discoveries. He also 
acknowledges his obligations to the Avorks of the late lamanted M. de Lamarck, and 
those of MM. de Blainville, Savigny, Fernssac, Des Heyes, D’Orbigny, Rudolphi, 
Bremser, Otto, Lenckart, Chamisso, Eisenhardt, Rang, Sowerby, Charles Desmou- 
lins, Quoy and Gaymard, Dclle Chiaje, Defrance, Deslonchamp, Audouin, Milne 
Edwards, Duges, Moquin Tandon, Morren, Ranzani, and other saAans 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. Exg. Ed. 
psi 
