FORMS 01' BEAXCHI^. 169 



French physiologist, Paul Bert, is perfectly right when he says 

 that any dispute as to whether this or that portion of the body 

 of an animal is its respiratory organ is fundamentally and 

 perfectly superfluous. But when special appendages are de- 

 veloped from the skin in a foliated or arborescent form — known 

 as gills — which seem specially adapted by their structure and 

 the delicacy of their walls to absorb more air from a given 

 body of water than the skin can, and to transmit it directly to 

 the blood or to the fluid of the body-cavity which circulates in 

 those gills, we are certainly quite justified in designating these 

 appendages as special organs of respiration. 



Such gills or branchiae occur in the intestine as well as on 

 the skin of the most dissimilar animals living in water. 



The gills of the outer skin bear so striking a relation to 

 the animal's mode of life that they must here be briefly dis- 

 cussed. In Fishes (fig. 47, a) and in many Amphibia the gills 

 are placed at the side of the head or partially under it, where 

 they are concealed beneath larger or smaller folds of the skin, 

 which, with the flat bones that support them, are known as the 

 gill-covering. In the embryo of the Shark (fig. 47, b, d) and of 

 Amphibia, external ramified gills appear before these internal 

 gills ; these, in the fishes, subsequently disappear, but in the 

 Amphibia persist throughout life {Perennihranchiata). In 

 Crustaceans we often find gills in places analogous to their posi- 

 tion in fishes, that is to say, by the side of the cephalothorax, 

 and covered by a large .skin-fold attached to it ; this is the case 

 in O'^abs (Bradhyura), and in many of the Macroura, Lobsters, 

 Prawns, &c. In other Crustaceans, on the contrary, as in our 

 Water-lice [Asellus) or the Sea-louse (Idotea), they are situated 

 at the end of the abdomen, and in yet other species they ai-e 

 appendages of the legs, whatever part of the body these may 

 be attached to. In the class of Mollusca we find not less than 

 five forms of gills morphologically difierent — first, the usual gills 

 of bivalves (fig. 48, h) ; secondly, those borne on the back of 

 many of the naked marine Mollusca, as the jEolidce, Doris, and 

 others (fig. 48, d) ; thirdly, the dorsal branchial cavities of such 

 genera as Neritina and Melania ; fourthly, lateral gills, as in 

 riyllidia; and lastly, the highly interesting mantle-gills of 



