ARTHROPODS AND MOLLUSCS 177 



which possess a lung-sac instead of gills constitute the order 

 Pulmonata. The pulmonate pond- and land-snails and 

 slugs are vegetable feeders, and where they occur in large 

 numbers do much injury to vegetation. While the common 

 pond-srrails have but one pair of feelers, at the base of which 

 are found the eyes, most of the land-snails and slugs have 

 two pairs of "horns," the eyes being on the tips of the second 

 pair. 



There are other snails common in ponds, also called, like 

 the pulmonate forms, pond-snails, which have gills and no 

 lung-sac. These pond-snails belong to a different order of 

 molluscs, and live on the bottom of the pond, crawling 

 about in the soft mud and feeding on animal instead of 

 vegetable food. 



The shells of the various kinds of snails vary much. In 

 many of the land-snails the spiral is not spire-shaped or 

 conical, but is flat. In some the whorls of the spiral run 

 from left to right (dextral) when the shell is looked at with 

 apex held toward one, while in others the whorls run from 

 right to left (sinistral). 



Of the hosts of marine Gastropods we can notice only a 

 few kinds. The nudibranchs are a group of beautiful forms 

 in which the shell is wholly wanting and the mantle is usually 

 absent. The gills are thus exposed and are usually in the 

 shape of delicate freely projecting tufts arranged in rows 

 along the back. The body is often strikingly and variedly 

 colored. These soft, naked "sea-slugs" live near the shore, 

 creeping about among the rocks and seaweeds. About a 

 thousand species of nudibranchs are known. 



Among the shell-forming marine Gastropods there is 

 great variety in the size and shape and coloring of the shells. 

 Many are beautifully colored and patterned; others are 

 oddly and fantastically shaped. The cowries, or porcelain 

 shells, familiar in collections of ocean curiosities, have a 

 large body whorl and a very short flat spire, and the brightly 



