516 



MOLLUSCA. 



THE ELEPHANTS TOOTH. 



TIIK DKN'TALIID^]. 



Tlu'se, which have been oalk'<l Tooth-Shells and Shell- Worms, have a tube-shaped shell, taper- 

 ing from one end to the other, and 

 slightly curved throughout, so as to 

 appear like a minute elephant's 

 tusk; hence the common species, 

 Dentalium elephantinum, is popu- 

 larly called -Elejjhanfs Tooth. It 

 has an aperture at each end ; the animal inhabiting it is of a cylindrical form, and inclosed in a 

 sac-liko mantle. Thirtv living and seventy fossil species are known. They are of various sizes, 

 and arc found on nearly all shores. 



THE OPISTHOBEANCHIATA. 



Tliis term is from the Greek opisthos, behind, and b)-anchia;, giWs. In the animals which com- 

 pose the irroup, the branohiie are not generally inclosed within a cavity of the mantle, but are 

 more or less exposed on the back or sides of the animal, generally toward the posterior portion; 

 and the auricle of the heart, which receives the blood from the gills, is placed behind the ven- 

 tricle. All the species are hermaphrodites ; few of them are inclosed in a shell ; some have 

 an internal shell, but the majority are naked. The three following families are called JVudi- 

 branchiata, the others Tccfibranchiata. 



THE DORID.E. 



In these animals, called Sea-Lemons, the body is shelless and oblong; the gills plume-like, and 

 placed in a circle on the middle of the back ; the tentacles two ; the eye-specks immersed behind 

 the tentacles, and not always visible in the adult. They are delicate little creatures, adorned 

 with pleasing colors, and are generally found along the sea-coasts, crawling upon sea-weed. 

 They vary in size from one quarter of an inch to three inches, are carnivorous, and feed on zoo- 

 phytes and sponges ; found on rocky coasts in nearly all parts of the world. 



THE TRITOXIID^. 



The Tritonias are naked, like the preceding, with plumose or 



papillose gills; the body is elongated, sometimes six inches long; 



some are found under stones at low water, and some floating on 



sea-weed. The T. Hombergii attains a length of six inches. 



TRiTONiA HOMBERGII. Thsrc arc several analogous genera. 



THE ^OLIDID^. 

 In these animals, which generally appear like small slugs, the branchiae are arranged along on 



each side of the back; the tentacles 

 are not retractile ; some species 

 have a very singular appearance, 

 the bodv beinar slender, and the 

 THE EOLis PAPiLLosA. gill-tufts supportcd at the extremi- 



ties of lateral footstalks. 



THE PLEUROBRANCHID^. 



These animals, called Umbrella- Shells, are furnished 

 with a shell, which, however, is often concealed by 

 the mantle ; the foot is usually large, and the feather- 

 like gill hidden between a fold of the mantle and the 

 foot. The shell is sometimes limpet-shaped. The 

 Umbrella Mediterranea and the U. Indica are the 

 only known species. They are found on rocky coasts, 

 at various depths from the surface to thirty fathoms. 



THE MEDITERRANEAN CMBRELLA-SHELL. 



