Sense Organs of Molluscs, 89 



Pteropoda. The third sub-class includes all the re- 

 maining forms, a few of which are free, swimming 

 with the foot flattened into a screw-propeller. Most 

 of them crawl on the under surface of their body, and 

 hence are called Gasteropoda. Among these, a large 

 number are branchiateTor gill-breathing ; these make 

 up one order ; the others are pulmonate or air-breath- 

 ing, and make up a second order. The branchiate 

 forms have the gills either in front of the heart or else 

 behind the heart, as in the great group of shell less 

 naked-gilled molluscs like the Doris or yEolis, so com- 

 mon on the shore. Some of the former sub-order 

 have shells of eight valves, like the common Chiton ; 

 others have the gills all round the body, under the 

 mantle, and equal on both sides, as in the limpets, 

 or they may be unsymmetrical as in the ear-shells, 

 cones, shoulder of-mutton shells, etc. 



Snails. The air-breathing order are the land shells 

 or snails ; they have their breathing chamber placed 

 behind the heart, and the larva has in general a very 

 rudimental ciliary lobe. A curious difference has 

 been noted between the gill -bearing and lung-bearing 

 molluscs, namely, that the intestinal tube is bent 

 towards the haemal side of the body, that is, towards 

 the heart, in the former, while it is turned towards 

 the nerve-ganglion in the latter. 



