In general, it may be said that the breathing process of mollusks 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 gasteropods that are worthy of notice. The gills themselves differ throughout the various orders of this class both as to their structure and their relative position; in one of the orders (the /Pulmonata/) 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 /Littorinidæ/, 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 [pg339] crowded with capillaries with exceedingly thin walls. This very remarkable method of surface respiration is proved to exist in the nudibranch or shell-less marine /Gasteropoda/, for they possess neither gills nor lungs. OSPHRADIUM