338 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- 
monata) the gills have entirely disappeared, and are replaced by 
a pulmonary sae 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 
Inttorinide, 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 
