STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA, ol 
rously. The most generally prevalent nucleus appears to be 
the bodies or eggs of minute internal parasites, such as /ilaria, 
distoma, buchephalus, &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. 
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Fig. 21. Pinna. si 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.{ 
The jibrows 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 yery thick (as in the fossil tnoceramus and trichites), will 
break up vertically into fragments, exhibiting on their edges a 
structure lke 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 (pholadide) the carbonate of hme 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 ageregated.§ 
* Drs. Mobius and Kelaart, Annals of Nat. Hist., i., 1858, p. 81. 
7 Figs. 21, 22,23. Magnified 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. 
{ They are pink in turbinellus and strombus; white in estrea; white or glassy, 
purple or black, in mytilus ; rose-coloured and translucent in ponna.—( Gray.) 
§ The specific gravity of floating shells (such as argonauta and ianthina) is lower 
uhan that of any others.—(De la Beche.) 
