84 ZOOLOGICAL PHILOSOPHY 



ruricola) which habitually live on land and breathe air quite naturally 

 with their gills. Eventually this habit of breathing air with gills 

 became a necessity to many molluscs which acquired it : it even 

 modified the organ in such a way that the gills of these animals, 

 having no further need for so many points of contact with the respired 

 fluid, became adherent to the walls of the cavity which contains 

 them. 



As a result we may distinguish among molluscs two kinds of 

 gills. 



The first kind consist of networks of vessels running through the 

 skin of an internal cavity which is not protruded and can only breathe 

 air : these may be called aerial gills. 



The second kind are organs nearly always protruded either within 

 or without the animal and forming fringes or pectinate lamellae or 

 edgings, etc. : these can only achieve respiration by means of the 

 contact of fluid water, and may be called aquatic gills. 



If the differences in the habits of animals produce differences in their 

 organs, it will be useful in describing the special characters of certain 

 orders of molluscs to distinguish those which have aerial gills from 

 those whose gills can only breathe water ; but in any case they are 

 always gills and it appears to us quite improper to say that the molluscs 

 which breathe air possess a lung. How often the abuse of words 

 and wrong applications of names have served to distort objects and 

 lead us into error ! 



After all, is the difi'erence so great between the respiratory organ 

 of Pneumoderma, which consists in a vascular network running over 

 an external skin, and the vascular network of snails, which runs over 

 an internal skin ? Yet Pneumoderma appears to breathe nothing but 

 water. 



Let us further enquire for a moment if there are any affinities between 

 the respiratory organ of air-breathing molluscs and the lung of verte- 

 brates. 



A lung is essentially a peculiar spongy mass composed of more 

 or less numerous cells into which air is always entering in nature. 

 The entrance is effected through the animal's mouth and thence by a 

 more or less cartilaginous canal called the trachea, which usually 

 sub-divides into branches known as bronchi, culminating in the cells. 

 The cells and bronchi are alternately filled and emptied of air by 

 successive swellings and shrinkings of the cavity of the body contain- 

 ing the mass ; so that distinct alternate inspirations and expirations 

 are characteristic of a lung. This organ can only tolerate the contact 

 of air and is highly irritated by water or any other material. It is 

 therefore different in character from the branchial cavity of certain 



