PHYSIOLOGY OF THE INVERTEBRATA. 277 



the horny cuticle which spreads over the external surface of 

 the valves, and is reflected over the ventral edges into the 

 mantle or pallium. 



The pearly or nacreus layer has a laminated texture, and 

 is secreted by the mantle. The production of pearls (e.g., in 

 Meleagrina margaritifera, the 

 "pearl oyster") is as follows: 

 A grain of sand, or other hard 

 substance, gets in between the 

 pallium and the shell. Con- 

 sequently the external surface 

 of the pallium becomes irri- 

 tated, and the laminated mo- X^* 

 ther-of -pearl layer (nacreus -^ig 

 layer) is secreted by the pal- section of shell of Gaper. 

 Hum, during the remainder of « = cuticuia. b = prismatic layer. 



the animal's life, around this <" = "acreus layer, d = epithelium. 

 ■ ., , 1 » « = mantle. 



irritant nucleus.* 



The exoskeletons of the Brachyura and Macroura have a 

 similar structure to the Lamellibranch shells;! and it has 

 been shown that the particular combinations of lime requisite 

 for the formation of these shells, &c., are calcium chloride, 

 calcium carbonate, or calcium phosphate. The sulphate of 

 lime present in sea water cannot be utilised for shell formation 

 unless it is first converted into one of the above forms. The 

 researches of Irvine and Woodheadj prove that " shell forma- 

 tion in the crab is somewhat different from egg-shell formation 

 in the hen, and occupies an intermediate position between 



* According to Dr. G. Harley, F.R.S. (Proe. Hoy. Soc, 1888) pearls have 

 the^following composition : — ■ 



Calcium carbonate 9l'72 



Organic matter (animal) .... 5.94 

 Water 2.23 



99.89 



t See Vitzou's paper in Archiv de Biologic, tome 10, p. 659. 

 t Proc, Boy, Soc. Edin., vol. 15, p. 308 ; vol. 16, p. 324. 



