STEUCTUEE AND PHYSIOLOGY OP THE MOLLTJSCA. 



31 



rously. The most generally prevalent nucleus appears to be 

 the bodies or eggs of minute internal parasites, such as filaria, 

 distoma, huchephalus, &c.* Completely spherical pearls can 

 only be formed loose in the muscles, or other soft parts of the 

 animal. The Chinese obtain them artificially by introducing 

 into the living mussel foreign substances, such as pieces of 

 mother-of-pearl fixed to wires, which thus become coated with 

 a more brilliant material. 



Fig. 21. Pinna. 



Fig. 22. Terebratula. 



Fig. 23. Pearl, t 



Similar prominences and concretions — pearls which are not 

 pearly — are formed inside porcellanous shells; these are as 

 variable in colour as the surfaces on which they are formed. J 



The fihrous shells consist of successive layers of prismatic 

 cells containing translucent carbonate of lime ; and the cells of 

 each successive layer correspond, so that the shell, especially 

 when very thick (as in the fossil inoceramus and trichites), will 

 break up vertically into fragments, exhibiting on their edges a 

 structure like arragonite, or satin-spar. Horizontal sections 

 exhibit a cellular network, with here and there a dark cell, 

 which is empty (Fig. 21). 



The oyster has a laminated structure, owing to the irregular 

 accumulation of the cells in its successive layers, and breaks 

 up into horizontal plates. 



In the boring-shells (pholadidce) the carbonate of lime has an 

 atomic arrangement like arragonite, which is considerably 

 harder than calcareous spar ; in other cases the difference 

 in hardness depends on the proportion of animal matter and the 

 manner in which the layers are aggregated. § 



* Drs. Mobius and Kelaart, Annals of Nat. Hist, i., 1858, p. 81. 



t Figs. 21, 22, 23.' gMagnified sections of shells, from Dr. Carpenter. Fragments of 

 shell ground very thin, and cemented to glass slides- with Canada balsam, are easily 

 prepared, and form curious microscopic objects. 



t They are pink in turbinellus and strombus; white in rstrea; white or glassy, 

 purple or black, mmytilus ; rose-coloured and translucent in pinna. — ( Gray,) 



§ The specific gravity of floating shells (such as argonauta and ianthina) is lower 

 than that of any others.— (Z)e la Beche.) 



