LAMELLIBRANCHIATA. 25 



afterwards ; while the heart, kidney, and gills are still later in 

 making their appearance. Among the provisional arrangements the 

 velum, which proceeds from the sides of the praeoral ciliated ring, is 

 very generally present, and in the free-swimming larvae has the form 

 of a large ciliated ring or collar. 



The development of the freshwater forms (Cydas, L T nio, Anodonta), 

 in which the eggs and embryos are contained in well-protected 

 brood pouches, may generally be called direct. The marine Lamelli- 

 branchs, on the other hand, are born at an early stage, and swim 

 about for a long time as larvae with large umbrella-like velum, from 

 which the labial palps are developed (fig. 503). 



The Lamellibranchs are for the most part marine and live at 

 different depths, sometimes creeping, sometimes swimming and 

 j umping. Many are without the power 

 of changing their position, inasmuch 

 as they fix themselves at an early age 

 by means of the byssus threads to 

 rocks and stones (oysters). Others, 

 as the boring forms, bore passages in 

 the wood of ships and piles and in 

 rocks. 



The Lamellibranchs had a wide dis- 

 tribution in the earlier periods of the 

 earth's history, and their fossil shells 

 are most excellently preserved ; they 



, -i ji , FIG. 503. Larva ofMontacuta bidentata 



are therefore of the greatest import- (after Lov , n) . s> velum . Sp> apical 



ance as characteristic fossils for the plate with flagellum ; D, intestine ; 



j, ,. c .-, t. e .. L , liver ; SM, anterior adductor mus- 



aetermination or the age or formations. cle . Pet f oot- 



1. ASIPHONIA. 



Mantle without siphons. Pallial impression simple. 



Fam. Ostreidae. Oysters. Shell valves unequal, laminated, with weak hinge 

 usually without teeth, and simple central adductor muscle. In the true oysters 

 the more arched left valve is firmly attached, while the right and upper valve, 

 which is fastened by an internal ligament, lies as an operculum on the lower 

 valve. Mantle completely split and fringed at the edge ; gill lamellae, on the 

 contrary, partially fused on their outer edge. Foot absent or rudimentary. 

 They usually live together, like colonies, in the warmer seas, where they may 

 form banks of considerable extent (oyster banks). They were also represented 

 in earlier times, especially in the Jura and in the Chalk. Ostrea edulis L., 

 oyster, on the coasts of Europe on rocky ground ; probably includes a seriei; of 

 different species according to the locality. According to Davaine, the oysters 

 are said to produce only male sexual products towards the end of the first year, 



