3J8 MARINE INVERTEBRATES 



(as in the ideal mollusk ) ; but the presence of two sets of 

 gills only indicates, in these gasteropods, that the twisting process 

 of the viscera, already alluded to as so remarkable a feature of 

 this class, has not proceeded far enough to destroy the functional 

 activity of one of the gills. 



In general, it may be said that the breathing process of mol- 

 lusks does not markedly differ from the respiratory methods of 

 higher forms of animals. Whether the breathing is effected by 

 gills or by a pulmonary sac, the essential features of the system 

 are the same as in fishes or mammals. There are, however, some 

 rather peculiar developments in the breathing system of gaster- 

 opods that are worthy of notice. The gills themselves differ 

 throughout the various orders of this class both as to their struc- 

 ture and their relative position ; in one of the orders (the Pul- 

 monatci) the gills have entirely disappeared, and are replaced by 

 a pulmonary sac which fulfils the purpose of a lung. 



It is a generally accepted theory that all land or air-breathing 

 mollusks were originally marine, but by a gradual change in their 

 habits have developed into terrestrial forms, losing their gills and 

 acquiring in their place organs which correspond to lungs. The 

 theory is supported by the fact that many species of marine 

 mollusks live between tide-marks, and for several hours daily are 

 obliged to maintain life outside of their native element. The 

 Littorinidce, which may always be found in abundance on both the 

 east and the west American coasts, usually live high above low- 

 tide line, and some tropical species of this genus actually live in 

 the trees above the reach of high tide. They have gills just as 

 in Buccinum or Fulgur, but the Littorina shell is strongly suggestive 

 of certain land forms, and there is reason to believe that the genus 

 is in course of evolution into a pulmonate. On the other hand, 

 there are to be found in fresh water numerous genera which are 

 true pulmonates, and which are obliged to come to the surface 

 every few minutes to obtain air. One notable genus of marine 

 gasteropods possesses both lungs and gills, and one marine 

 form has entirely lost its gills and breathes only by means of 

 lungs. It is quite likely that all mollusks are able to breathe 

 more or less all along the exposed mantle surface, which is usually 



