302 MOLLUSC A. 



ber 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 re- 

 gards the structure of the latter ; for some of them respire 

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

 gans, however, and those of locomotion, are generally arranged 

 symmetrically on the two sides of an axis(l). 



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

 their pulmonary circulation describes a separate and distinct 

 circle. This function is at least always aided by a fleshy ven- 

 tricle, 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 Cephalopoda alone are provided besides \Yith a pulmo- 

 nary ventricle, which is even divided into two. The aortic 

 ventricle is also divided in some genera, as in Area and Lin- 

 gula; 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 fre- 

 quently 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 



(1) N.B. LiniiKus united all invertebrate animals without articulated limbs in 

 a single class, under the name of Vermes, dividing them into five orders: the 

 iNTESTijfA, embracing some of my Annelides and Intestina; the Moilusca, com- 

 prehending my Naked Mollusca, my Echinodermata, and part of my Intestina and 

 Zoophytes; the Testacea, comprising my Mollusca and Annelides ivith shells.- the 

 Ltthophtta, or Stony Corals; and the Zoophytes, embracing the remainder of 

 the Polypi, some of th^ Intestina and the Infusoria. 



No regard whatever was paid to nature in this arrangement, and Brugiere, 

 Encycl. Method., endeavoured to rectify it. He there established six orders of 

 worms, viz. the Infuhiosa; the Intestina, including the Annelides; the Mollus- 

 ca, uniting several of my Zoophytes to my true Mollusca; the EcniNODERMAXA, 

 which only comprised Echinus and Asterias; the Testacea, nearly the same as 

 those of Linnseus; and the Zoophxtes, under which name he included the Co- 

 rals only. This arrangement was merely superior to that of Linn^us in the more 

 complete approximation of the Annelides, and by the distinction it effected of a 

 part of the Echinodermata. 



r proposed a new arrangement of all the invertebrate animals, founded on their 

 internal structure, in a paper read before the Societe d'Histoire Naturelle on the 

 10th of May 1795, of which my subsequent labours on this part of natural history 

 are the development. 



