13G HABITS AND ECONOMY OF THE MOLLUSCA. 



alternating generation, one of the forms being the echinoderm, 

 the other the niollusk ; but this theory is not tenable, and it is 

 certain that Entoconcha is a gastropod, but whether related to 

 Eulima, or whether the larva of a nndibranchiate, has not been 

 determined. Some species of Odostomia are scarcely obtained 

 elsewhere except fixed to the ears of the valves of Pecten. 

 They may live on the secretions of that molkisk, or possibly 

 upon minute animals found upon the valves. 



Magilus, Leptoconcha, Cryptobia, Coralliophila, Rhizochilus, 

 Pedicularia, etc., live among polyps and on Alc3^onaria ; and 

 many Eolidians remain attached to Hydrozoa and Bryozoa. 

 Lamellaria perspicua feeds on Leptocliuum, one of the com- 

 pound ascidians, and deposits its eggs in a cavity which it digs 

 in the colonies of these animals. 



Among bivalves, the European species Montacuta suhstriata is 

 found attached to the spines of sea-urchins ; the Lepton para- 

 siticum, of Kerguelen's Island has been found in the vicinity of 

 the mouth of a Hemiaster (Dall) ; ModiolatHa marmiorata is 

 nearl}^ always encysted in the teguments of simple ascidians ; 

 Vulsella, and Crenatula and a number of species of other genera 

 live in sponges ; Tridacna and Pedum on coral. 



In fresh waters, young Unios and Anodons fix themselves on 

 the opercula, the lips and fins of fishes (Leuciscus, Gobio). Their 

 larval byssus and the spiny border of their larval shells favor 

 this parasitism. The true nature of these young mussels was at 

 first mistaken, and they were described under the name of 

 Glochidium parasiticum. 



Food. The food of the mollusca is either vegetable, or animal. 

 Nearly all the land snails are vegetable-feeders, and their depre- 

 dations are but too well known to the gardener and farmer in 

 Europe where many a crop of winter corn and spring tares has 

 been wasted by the ravages of the " small gray slug." In the 

 drier climate of the United States where the snails are mostly 

 compelled to seek th-e shade and dampness of forests, they are 

 seldom destructive to crops. They have their likings, too, for 

 particular plants, most of the pea-tribe and cabbage-tribe are 

 favorites, but they hold white mustard in abhorrence, and fast 

 or shift their quarters while that crop is on the ground. Some, 

 like the " cellar-snail," feed on cryptogamic vegetation, or on 

 decaying leaves ; and the slugs are attracted by fungi, or any 

 odorous substances. The round-mouthed sea-snails (holostomata) 

 are nearl}^ all vegetarians, and consequently limited to the shore 

 and the shallow waters in which sea-weeds grow. Beyond fifteen 

 fathoms, almost the only vegetable production is the nuilipore ; 

 but here corals and horny zoophytes take the place of algee, and 

 afford a nutritious diet. 



The whole of the bivalves live usually'- on infusoria, or on 



