BIONOMIC CHARACTERS OF MOLLUSCA 1019 



• 

 again, live in shallow water, even if stagnant, and will not stand 

 a long exposure. 



Various species of Neritina (A^. dnbia, A\ cic:;ac, etc.) live 

 habitually high up on the trees of the mangrove swamps, deposit- 

 ing their eggs, however, on the surface of the water. Others occur 

 on the dry land, far from any water. (Semper-26.) 



A'assa ohsoleta covers the gentle muddy beaches at low tide, 

 where dead organisms remain for it to feed upon, and it also 

 abounds on every exposed mud flat on our northern coast. Littorina 

 rudis and L. palliata are commonly found on the New England 

 shore clinging to rocks or to the stems of the marsh-grass (Spar- 

 tina), high up, where they are exposed to the air for half the day. 

 The lack of a siphon forces these animals to live above the mud 

 {12:168). On the marshes of Cold Spring Harbor, these species 

 of Littorina occur in places where they are "submerged for only 

 a short time at high tide, and then under water that is nearly fresh." 

 (Davenport-i2:7dp.) In the Mississippi sound Davenport found 

 nearly all of the individuals of Littorina irrorata "living on the 

 stems of the short marsh-grass twenty to thirty centimeters above 

 the water level and exposed to the sunlight." Littorina rudis lives 

 prevailingly where it is much exposed. On the English Channel it 

 has been found two meters above the other marine animals, where 

 it is moistened only by the highest tides (Fisher-13 r/i'i') ; and on 

 the New England coast it is sometimes found in similar positions. 

 Species of Littorina are reported as passing the winter out of water, 

 with their gill chambers full of air. (Simroth-27:5.^.) 



The majority of gastropods are shallow-water forms, though a 

 number of them range to depths of between 1,000 arwl 2,000 

 fathoms. The deep-sea gastropods are characterized by faint col- 

 ors, though often this is counterbalanced by the brilliancy and 

 beauty of the iridescence, and even the non-iridescent abyssal spe- 

 cies give out "a sort of sheen which is wanting in their shallow- 

 water allies." (Agassiz-i, ii:d?.) The coarse ornamentation by 

 knobs, spines, etc., so common in shallow-water species, does not 

 occur in the deep-sea forms, where the ornamentation is more 

 delicate, and often of exquisite richness and beauty. Gastropods 

 feeding on vegetable matter are wanting in the deep sea, where no 

 vegetable matter occurs, except what is brought down as sediment. 

 The food of deep-sea molluscs is largely confined to soft-tissued 

 animals, since thick shells and other hard armors are generally ab- 

 sent in these depths. Agassiz states that the Pleurotomidae out- 

 number any other group of molluscs in the abyssal fauna. These 

 gastropods are characterized by a notch in the outer lip near the 



