304 LOWER INVERTEBRATES. 



The pulmonary sac is formed by tlie union of the edge of tlie mantle to the body, 

 leaving a small round or oval entrance to a large sac, richly supplied with blood-ves- 

 sels, lu most of the order this lung serves for breathing air, even in the aquatic 

 forms. The operation can readily be witnessed in such a form as Linmcea, when kept 

 in confinement. At nearly regular intervals the snail will creep to the surface of the 

 water, and force a bubble of air out of the respiratory orifice, then more air is taken 

 in, and the snail descends .again to its pastures. Recent investigations on the Limneans 

 from the profound depths of some of the Swiss lakes have shown some interesting 

 features in connection with this lung. Of course, snails living at these great depths 

 could not ascend to the surface for a supply of air, and it was found that they had 

 acquired a capacity of breathing the oxygen contained in the water, although no gills 

 were present. It is also interesting to observe that all the fresh-water pulmonates 

 fill their lung sac with water in the younger stages, and only later do they adopt 

 the aerial respiration. The jjulmonary sac also subserves another function ; it 

 forms avery effective hydrostatic apparatus, as, by variations in its size, and con- 

 sequently in the amount of air, these animals are enableil to rise or sink in the water 

 at will. 



The opening to the pulmonary chamber is, of course, at the edge of the maiitle and 

 hence, as this part of the body is subject to considerable variation in size, the res]nra- 

 tory opening is far from constant in position. In the common snails (-Se&), for in- 

 stance, it is on the right side of the body, just within the shell as the animal is crawling 

 along, while in Testacella, where the mantle is vei-y small, it is found near the posterior 

 end of the body. Even the side of the body on which the resjiiratory orifice is placed 

 is not constant; in most it is found on the right side of the liody, Init occasionally it 

 occupies the median line behind. 



Even greater variations are found in the shell. In some it is lai'ge enough to contain 

 the whole body when retracted, some have it reduced to a scale-like plate on the sur- 

 face, others have it small and internal, while in still 

 others it is entirely absent. Usually it is coiled in a 

 spiral the whorls of which are dextral (revolving from 

 left to i-ight) but in some genera the reverse is the 

 case and the shell is wound from right to left. Even 

 in those which are normally dextral, sinistral mon- 

 strosities occur. It is a common idea, but an utterly 

 erroneous one, that the shells north of the equator are 

 coiled in one direction, and south of that line in the 

 ^"'i^kiila-i^Su^T^IS^tri^^''' pther, the coil of the shell following the snn. A little 

 investigation of our fresh-water shells will jiroduce 

 evidence utterly contradicting this. Only in the family Amphibolidre is an operculum 

 formed, but the other members of the order secrete a mucus which hardens and tightly 

 seals the aperture of the shell. This is known as the epiphragm and is formed when 

 the animal retires in winter or in a season of drought. On the return of moist weather, 

 it is broken down and the snail resumes its wanderings. In ClausiUa this epijjhragm 

 is a jiermanent structure and is fastened to the mouth of the shell by an elastic stalk, 

 so that it works as a spring door. 



The lingual ribbon is short and broad, and is armed with rows of very numerous 

 teeth, there being sometimes over two hundred in a single row. Each tooth has a 

 broad base and acute or denticulated recurved tips. This ribbon is opposed in some 



