148 DR. H. LYSTER JAMESON ON THE [Mar. 4, 



pearl are, like the shell, formed by the calcijfication of the cuticle 

 of the living cells, and owe their structure to the special characters 

 of that membrane or of the underlying epidermis. 



A section of a decalcified pearl shows the nucleiis, in which the 

 cuticle and sometimes the suckers of the Distomum can be 

 distinguished. Occasionally the outlines of the soft parts (e. g., 

 pharynx and digestive caeca) are still visible, as in PI. XIV. fig. 1, 

 ph. & dig. More generally, however, nothing can be seen but a 

 mass of yellowish-brown granular substance surrounded by the 

 cuticle (text-fig. 22). 



There is often a certain amount of refractive granular matter 

 associated with the remains of the worm, probably an excretion ; 

 and, if the parasite migrates out of the sac, this may form the 

 inconspicuous nucleus of a pearl. 



Just as the peripheral parts of a pearl present, when ground 

 down to a thin section, a similar structure to that of the shell, 

 so the conchyolin basis of a decalcified pearl shows the same 

 characters. The outermost layer of the latter is uncalcified and 

 continuous with the cuticle of the cells of the sac, just as the 

 outer mantle- epidermis is attached to the inner surface of the 

 shell (PI. XIY. fig. 1, con.'). 



There is no organic union between the conchyolin and the 

 nucleus. 



The sac containing the pearl is composed of a simple columnar 

 epithelium (PI. XIV. fig. 1 & text-fig. 22, s.), which in its histo- 

 logical structure, as well as in its power of secreting as a cuticle 

 the conchyolin basis of the pearl, is indistinguishable from the 

 outer epidermis of the mantle. 



Blood- spaces, containing corpuscles, are well developed around 

 the sac. 



Such a pearl cannot then be compared — as some writers have 

 suggested — with the concretions or calculi of cholesterin or other 

 substances found in the vertebrate body, but rather with the 

 structures sometimes found in epidermoid tumours and atheroma 

 cysts. 



Oi'igin and Development of the Pearl. 



The Trematode enters Mytilus edulis as a tailless Oercaria, and 

 at first may often be found between the mantle and shell. It is 

 probable that it reaches this position by boring through the 

 mantle, but I have not yet been able to find one in the act of 

 doing so. The larvae creep about on the inner surface of the 

 shell, and, after a v/hile, again enter the connective tissue of 

 the mantle, where they come to rest, assuming a spherical form. 

 They seem to avoid the more muscular parts of the mantle — no 

 doubt because the absence of a definite boring apparatus makes 

 it diificvilt for them to pass thi'ough the latter. When embedded 

 in the tissues they are visible to the naked eye as little yellowish 

 spots, about I mm. in diameter. 



At first the worm only occupies a space lined by connective- 



