1902.] ORIGIN OP PEARLS. 149 



tissue fibrils (PI. XIV. fig. 2), but soon the tissues of the host give 

 rise to an epithelial layer, which lines the space and ultimately 

 becomes the pearl-sac (PL XV. fig. 5, s.). 



This epithelium appears to arise quite independently of the 

 outer epidermis, and is no doubt due to a specific stimulation on 

 the part of the parasite, as other parasites, e. g. Sporocysts, Oestode 

 larvae, &c., are not surrounded by such a sac. 



At first a few cells appear (PI. XIV. figs. 2, 3, pr.), which pro- 

 liferate and arrange themselves along the walls of the cavity. 

 These cells are larger than the connective-tissue corpuscles, and 

 more susceptible to stains. They are flattened and polygonal in 

 surface view. Their nuclei (PI. XIV. fig. 3, n.) are large and 

 spherical, and show the conspicuous chromatin reticulum and 

 distinct nucleolus that characterize the nuclei of embryonic or 

 rapidly dividing tissues. 



I have not been able to find the nuclei of these cells actually 

 undergoing division. The proliferating sheet of cells ultimately 

 surrounds the parasite and becomes the sac. From the first 

 these cells are basally continvious with fibres of connective tissue 

 (PI. XIV. fig. 3, c.t.). Then- transformation into the pearl-sac is 

 a gradual one, and every step can be traced in sections of the 

 parasites in situ. 



If the Trematode larva completes its maximum possible term 

 of life it dies, and the tissues of the body break down to form a 

 structureless mass, which retains the form of the parasite owing 

 to the rigid cuticle. 



In this mass arise one or more centres of calcification (PL XVI. 

 fig. 8), and the precipitation of carbonate of lime goes on until 

 the whole larva is converted into a nodule which has the calco- 

 sphseritic structure already described for the nucleus. The 

 granular matter surrounding the worm, if present, also undergoes 

 calcification. 



The epithelium of the sac then begins to shed a cuticle of 

 conchyolin (PL XIV. fig. 1), and from this point the gxowth of 

 the pearl probably takes place on the same lines and at the same 

 rate as the thickening of the shell. 



The sac sometimes begins to form pearly substance before the 

 worm is completely calcified (PL XVII. fig. 16). 



The Distomid larvee sometimes leave the sac formed around 

 them, and voluntarily migrate into other parts of the body 

 before again settling down. Empty sacs may be found in the 

 mantle, and old specimens of the larva (distinguishable from 

 recently immigrated ones by their darker coloui- and laden 

 excretory organs) sometimes occur free between the mantle and 

 the shell. 



The occurrence of pearls in which the nucleus is not a Trematode 

 but merely a few refractive granules (PL XVII. fig. 13) can be 

 accounted for in this manner. 



Some compound pearls are evidently formed by shoi-t migrations 

 on the part of the Oercarise, which leave a small amount of 



