344 



LOWER INVERTEBRATES. 



off by a calcareous deposit, thus giving a ready means of separating the shell of the 

 mollusc from that of the worm. In Vermetus the shell is entire, but in SiliqtmHa 

 the tube has a longitudinal slit. Ccecum is usually placed here, but it differs 

 greatly in its lingual dentition, only two lateral teeth on either side being present, and 

 the rhachidians are absent. The shell begins as a spiral, but with growth the older 

 portions are lost and the shell becomes simply a curved tube embracing an arc of from 

 sixty to ninety degrees. Both Vermetus and Ccecum are represented on the shores of 

 the United States ; the species of the latter genus are very minute, and in assorting the 

 contents of the dredge they readily escape notice. Some of the Vei-metidae become 

 attached to sub-marine objects, while others are always free. Specific limits are not 

 very well defined, owing to the irregularity of the shells. 



The apple-shells of dealers belong to the family AMPtJLLAEiiD^. These live in 

 tropical countries, where they replace in the ponds and marshes the Paludinidae of 

 moi'e temperate climes. Africa and South America seem to be especially favorable to 

 their growth, and from these regions we have the finest specimens. They are an am- 

 phibious group, living apparently equally well either in or out of the water, yet they 

 require a certain amount of moisture for their well being, and when their native 

 marshes dry up they burrow deep into the mud. They are well fitted by nature for 

 their amphibious life, for they have both lungs and gills. The lung cavity is placed 

 above that which contains the gills, but the two are connected by means of an opening 

 through the fleshy partition. This structure enables them to breathe either air or 

 water, and Professor Carl Semper, of Wiirzburg, who spent a long time in the Philip- 

 pines, frequently observed "that the Ampullarice breathe not only with both gills and 

 lungs, but they do so in regular alternation ; for a certain time they inhale air at the 

 surface of the water, forming a hollow elongated tube by incurving the margin of the 



mantle, so that the hollow surface is closed against 

 the water and open only at the top. When they 

 have thus sucked in a sufficient quantity of air, 

 they reverse the margin of the mantle, opening 

 the tube, into which the water streams. The 

 changes are tolerably frequent, once or twice in 

 a few minutes, depending probably on the tem- 

 perature." 



The apple snails are long-lived, and specimens 

 have been brought to northern climes hidden in 

 the hollows of logs of mahogany from Honduras, 

 cut no one knows how long before. Mr. Laidly, 

 who lived for many years at Calcutta, tried many 

 experiments with them, and some specimens 

 which were confined for five years in a drawer 

 were alive at the end of that period. 



The shells are large, thin, and globular, cov- 

 ered with a glossy ej)idermis, and closed by a 

 horny operculum. The eyes are good-sized, and 

 are placed on short stalks, and the tentacles are very long and slender. The eggs are 

 large and spherical and are laid upon the stems of water-plants in large bunches, fur- 

 nishing many a meal for the marsh birds. The most prominent genus is AmpuUaria, 

 in which the epidermis is usually green, and the respiratory tube long. The South 



Fig. 442. — AmpuUaria canaliculas, apple snail. 



