1902.] ORIGIN OF PEARLS. 147 



epithelium of the pearl-sac acquires the special characters of the 

 adjacent part of the epidermis. 



Blisters. — It is proposed to confine this term to internal 

 excrescences of the shell, which are caused by the intrusion of 

 foreign bodies between the mantle and the shell, or by the secre- 

 tion of a nacreous cicatrix to close the perforations of boring 

 molluscs, worms, or sponges. These are sometimes referred to 

 as " attached pearls " or even as " pearls," but have a totally 

 different mode of origin and should never be confused with the 

 latter. 



Gonc7'etions. — In many molluscs small free calcosphferitic bodies 

 occur at times in the connective tissues, which, not being enclosed 

 in epidermal sacs, cannot acquire the structure of the shell- 

 substance. They are probably due to different causes in different 

 molluscs. In Tapes they are frequent, and are due to the 

 calcification of degenerated Sporocysts or of dead Cercariie con- 

 tained in the same. Similar concretions, which I found in 

 Pholas Candida at Billiers, were caused by dead Cercarise of 

 another species, contained in Sporocysts. 



Old examples of Mytilus edulis L., Modiola modiolus L., 

 Hippopus hippojius L., Margaritifera vulgaris (Schumacher), and 

 Anodonta sometimes contain similar bodies, but their origin in 

 these cases is uncertain. 



In all instances that have come under my notice they are more 

 or less sphei'ical, and composed of needle-like prisms of carbonate 

 of lime radiating from a centre. 



Strtocture of Pearls. 



A M7/tilus-'peiir\ examined entire often shows a darker spot in 

 the centre, which cori-esponds to the " nucleus." The nucleus is 

 always visible in a section ground from the pearl, though its size 

 vai'ies from "1 mra. to '7 mm. 



It is often yellowish brown or black, the colour being imparted 

 by the dead remains of the Trematode, or by the small amount of 

 residual matter left, if the worm has escaped from the sac 

 (PI. XVII. figs. 12-16). The crystalline structui-e of the nucleus 

 is quite different to that of the remainder of the pearl and to that 

 of the shell. We find in the nvicleus one (PI. XYII. fig. 16) or 

 more (PI. XVI. fig. 8, PI. XVII. fig. 14) centres of calcification, 

 consisting of spherical masses of radially ai-ranged crystals. Each 

 centre of calcification, if more than one be present in the nucleus, 

 shows a distinct black cross when viewed between crossed nicols. 

 Sometimes the nucleus is ii-regularly or incompletely calcified 

 (PI. XVII. figs. 14-16). The resemblance which concretions 

 and the nuclei of pearls bear to Harting^s bodies (Harting, 1872) 

 is interesting ; for they are formed, so far as we can judge, 

 in a similar manner, namely, by the slow precipitation of 

 carbonate of lime in a viscous substance like albumen or decaying 

 animal matter. On the other hand, the peripheral parts of the 



