PHYSIOLOGY OF THE INVERTEBRATA. 237 



In the Pteropoda "the delicate lining membrane of the 

 pallial cavity serves as the respiratory organ." 



The mantle is also " an accessory organ of respiration, 

 being so modified as to direct, or to cause, the flow of currents 

 of water over the branchite contained in its cavitv." 



In the Pidmogasteropoda (air-breathers) " the lining wall 

 of the mantle cavity becomes folded and highly vascular, 

 and subserves the aeration of the venous blood, which flows 

 through it on its way to the heart. The lung is here a 

 modification of the integument, and might be termed an 

 external lung The lungs of the air-breathing Vcrtebrata^ 

 on the contrary, are diverticula of the alimentary canal ; 

 .... and the blood flows from the heart." 



The membranous respiratory sac of the Pidmogastcropoda 

 is not morphologically a true lung, as it is developed from 

 the integument ; but it may be ijhysiologically regarded as 

 a rudimentary lung performing a similar function to the 

 true Vertebrate lung which first appears in Pisces* 



" Many animals are truly amphibious, combining aquatic 

 and aerial respiratory organs. Thus, among Molluscs, Am- 

 indlaria and Onchidum combine branchiee with pulmonary 

 organs." (Huxley.) 



The Pulmogasteropod lung is the simplest form of lung to 

 be met with in the animal kingdom. It has been compared 

 to " a single so-called air-cell of the Mammalian lung. In 

 each case there is an internal cavity lined by a delicate 

 membrane supplied with impure blood, and having air in 

 contact with its free surface." 



Tissue-respiration is wonderfully well developed in the 

 MoUusmy for MacMunn has discovered an abundance of 

 respiratory pigments in these animals. 



In Udrcca, JJiiio, Anodonta, Mytilus, Livuiccus, Paludina, 

 Patella, Purjnira, Littorina, Hcli):, Limax, and Avion, histo- 



* In Lepidosiren (mud-fish) there is a transition from the piscine air- 

 bladder tu the Reptilian form of lung. The air-bladder of the fish appears 

 to be chiefly a hydrostatic organ, or rather an accessory one of respiration. 



