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 branchiate, or 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 ^Eolis, 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 

 in front of the heart, and the larva has in general a 

 very rudimentary 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. 



