LA5IELLIBRANCIIIATA. 521 



grating the chalk with its rasp-like valves, licking it up when pul- 

 verised with its foot, forcing it througli its principal siphon, and 

 squirting it out in oblong nodulos. They turn from side to side, 

 never going more than half-round, in their hole, and cease to work as 

 soon as the hole is deep enough to shelter them." The Pholades 

 attain their largest size in soft yielding stone ; whilst in hard, and 

 especially gritty rocks, they are dwarfed in size, and the rough 

 surface of their shell is worn away. M. Caillaud has shown that the 

 valves are quite equal to the work of boring in limestone, by imitating 

 tlie natural conditions as nearly as possible, and making such a hole 

 with them.* 



In Litliodomus, Saxicava, and Ungulina, the valves are smooth, 

 and retain the periostracum ; yet they bore into the hardest marble, 

 and still harder shells : their holes, like that of Clavagella, are not 

 cylindrical, and are doubtless formed, as in that genus, by the agency 

 of the thickened muscular borders of the pedial aperture. 



Teredo navalis bores in the direction of the grain, unless it 

 meets another Teredo, or a knot in the timber : they are probably 

 warned by their organ of hearing of such contiguity. The rasp-dust 

 is introduced by the foot into the pallial cavity, and is swallowed : 

 the long intestine of the ship-worm is usually laden with this debris. 

 The piles of the dikes in Holland received so much damage from the 

 ravages of the Teredo, in 1731-2, as to occasion serious alarm of a 

 destructive inundation of the country, as the consequence of the ope- 

 rations of this seemingly feeble vermiform mollusk. 



Generation of LamellibrancJis. — So far as actual experience ex- 

 tends, parthenogenesis is arrested in the molluscous province at the 

 Tunicated class. The Lamellibranchs propagate exclusively by im- 

 pregnated ova ; the individuals from which, after undergoing more 

 or less metamorphosis, exhaust the remnant of their developmental 

 force by completing the male or the female organs. Here, therefore, 

 there is no alternation of generative modes, save in so far as the 

 sexless state of the twin glochidium {Jig. 194., c) intervenes between 

 the impregnated ovum and the perfect male or female bivalves. 



The sexual organs of the higher Acephala, although as a general 

 rule developed separately on distinct individuals, present a very 

 simple and low character. Enormous in bulk, they ramify over the 

 mantle in some footless Lamellibranchs (Anomia, e. g.), as in the 

 Brachiopods, whilst the sperm-cells or the ova are aggregated at the 

 base and interior of the foot in those bivalves which have that 

 appendage. 



• CCCVII. p. 327. 



