236 



BIOLOGY. 



[Book ii. 



movement, and an uninterrupted current penetrates to the vicinity 

 of the stomach (Gegenbaur). 



In a small group of mollusks the pul- 

 monary organ appears. In these pul- 

 monate mollusks the lung is only a simple 

 sac, a cutaneous depression communicating 

 with the exterior by an orifice. Some, 

 as for instance the ampullaria, have at the 

 same time a branchia and a pulmonary 

 sac with contractile orifice. Of these 

 pulmonate mollusks some are terrestrial, 

 . the others aquatic ; and these last often 

 come to the surface of the water to 

 breathe. They need the free atmosphere, 

 and in a closed vessel, as Spallanzani saw, 

 they, without immediately suffering as- 

 phyxia, transform into carboniq acid the 

 whole of the oxygen contained in the 

 confined medium. 



The two chief respiratory modes, the 

 branchial mode and the pulmonary mode, 

 are observable in the vertebrates, but in a 

 higher degree of perfectionment. Here 

 the external tegument still respires but 

 accessorily, and the chief part of the 

 function is accomplished by special organs. 

 Nevertheless the respiratory apparatus is 

 always more or less connected with the 

 intestinal canal ; it seems to be a diver- 

 ticulum thereof. In fishes, in which the 

 branchial apparatus attains its maximtun 

 of development, it is supported by arcs 

 appertaining to the visceral skeleton (Figs. 

 31 and 32). The water penetrates by the mouth of the animal, 

 and is not expulsed till it has traversed the branchial clefts, and 



Fia. 3J. 



Respiratory organs of the 

 Myxi/ae glutinosa seen on 

 the ventral side ; o, oeso- 

 phagus ; i, internal bran- 

 chial canals ; hr, branchial 

 sacs : Br, external bran- 

 chial canals uniting in a 

 general branchial conduit 

 in s; e, oesophago-cuta- 

 neous canal ; a, auricle of 

 the heart ; v, ventricle ; 

 ab, branchial artery send- 

 ing a branch to each bran- 

 chia ;*d, wall of the body 

 thrown outward and back- 

 ward. 



