Febeuary 8, 1907] 



SCIENCE 



209 



tion by myxobolids increased growth in the 

 skin resulted. It is also a distance effect, 

 if not so great, when according to Leuckart 

 the epithelial cells in eoecidial nodules of 

 the rabbit liver increase strongly in num- 

 bers. Among the epizoa there are many 

 which produce such growths in the epithe- 

 lium. The various species of itch mites 

 excite a proliferation of the cells iintil there 

 are formed crusts or thickened masses of 

 considerable extent in which the galleries 

 of the mites are constructed. These pro- 

 liferations are said to be due merely to the 

 mechanical stimulus exerted by the mites, 

 and the crusts are formed by the addition 

 of serous exudates. Such formations are 

 thoroughly characteristic of the work of 

 these parasites. Less well known are other 

 cases of the same type such as that of gall- 

 producing copepods which infest actinia. 



But such stimulation is not always purely 

 indefinite. Parasites often produce un- 

 usual forms in the region of the host in 

 which they reside ; thus, Woodworth, ex- 

 amining a skull of the common skunk, 

 found that prominent swellings in the 

 frontal bones near the sagittal plane were 

 due to a nematode. The bone was ex- 

 tremely thin and in the subjacent frontal 

 sinus lay a nest of fifteen to twenty of these 

 parasites. The belief was expressed that 

 the prevalent frontal enlargements of other 

 American Mustelidte are probably due to 

 the same parasite. 



Perhaps the most common form of mor- 

 phological change on the part of the host is 

 the production of a cyst about the parasite. 

 It is composed in most instances, in part at 

 least, by the host animal, and consists in its 

 simplest form of an enveloping mass of con- 

 nective tissue. This reaction against the 

 invasion of the parasite is found in almost 

 all groups. 



The formation of pearls appears to be 

 due regularly, if not exclvTsively, to the 

 intrusion of parasitic larvae. The host re- 



sponds to the mechanical or chemical 

 stimiilus of the in-wandering larva by pro- 

 ducing an epithelial sac which surrounds 

 the parasite. Such a cyst formation by the 

 host very generally follows when a parasite 

 settles down to enter upon a resting stage 

 in the body of the host. But here the 

 character of the cyst wall leads normally to 

 the formation of a deposit of lime of the 

 same sort as the inner lining of the moUusk 

 shell and the larval parasite becomes the 

 nucleus of a pearl. This stimulus to pearl 

 formation is not given by any particular 

 species of parasite, but is traceable even to 

 members of different classes of animals. 

 Thus in European mussels pearls are 

 formed about the cercarife of Trematodes.* 



But in the Ceylon pearl oyster, which is 

 more nearly related to the mussels than to 

 the oyster, the formation of pearls is due 

 to certain cestode larvae which undergo a 

 portion of their development in the tissue 

 of the liver, gills and mantle of the pearl 

 oyster. Of these larvae, such as for some 

 yet unexplained reason do not succeed in 

 carrying out their life cycle, become im- 

 mured in the center of a pearl. Shipley and 

 Horuell, who have investigated wdth suc- 

 cess these phenomena, say "economically 

 these unpleasant little creatures are of su- 

 preme importance to the Ceylon pearl 

 fishery, as their presence in the oyster 

 causes the formation of the finest quality of 

 pearl and those of the highest luster." 

 Another economic factor may be merely 

 noted in passing. In 1859 Kelaart called 

 attention distinctly to the possibility of in- 

 fecting other beds with the larvas of the 

 pearl-producing parasites in order to in- 

 crease the quantity of pearls. Beds of 



' In one ease carefully elucidated by Jameson 

 the adult fluke inhabits the eider duck and the 

 scoter, its sporoeyst occurs in the cockle, while 

 the tailless eercaria is found in the mussel and 

 forms the nucleus of the small, lusterless pearls 

 of that species. Another trematode is the cause 

 of pearl formation in the fresh-water anodons. 



