448 PHILOSOPHY OF ZOOLOGY. 



The organs of respiration exhibit the two modification si 

 of lungs and gills, to enable us to divide the gasteropoda 

 into two classes, which we have termed Pulmonifera and 

 Branchifera. M. CUVIEII appears to have been in some 

 measure aware of the importance of the distinction when 

 he instituted his order Pulmones ; but he afterwards suffer- 

 ed himself to be more influenced by the presence of an oper- 

 culum, the shape of the aperture of the shell, and the sup- 

 posed separation of the sexes, than by the characters of the 

 respiratory organs. 



Class 1. PlJLMOtflFERA, 



The pulmonary cavity is single and lateral. Its orifice 

 is capable of being closed at the will of the animal. The 

 bloodvessels are spread, chiefly on the walls and roof, like 

 delicate net-work. The opening of the cavity is usually on 

 the right side, with the anus behind it, and the sexual ori- 

 fice is in the front near the head. In some of the genera, 

 these openings are situate on the left side. The shells of 

 the former are denominated dextral', of the latter sinistral. 

 This change in the position of the external openings is ac- 

 companied by a corresponding alteration in the arrangement 

 of the internal organs. The heart, for example, is always 

 placed on the side opposite the pulmonary cavity. In the 

 dextral shells, therefore, it is sinistral. In both kinds, 

 however, all the organs preserve the same relation to the 

 back and belly, the head and tail. It is impossible, there- 

 fore, to conceive a dextral animal changed into a sinistral, 

 by any circumstance which could take place at the period 

 of hatching, as M. Bosc was inclined to believe. This ar- 

 rangement of the organs must have been not merely conge- 

 nital, but coeval with the formation of the embryo. In 

 some species all the individuals are sinistral, while in others 

 the occurrence is rarely met with in a solitary example, 



