April 27. 1S83. 



SCIENCE. 



331 



may be- necessary. The outer ciliated cells swell up, 

 and are finally cast off. Tlie embryo then becomes an 

 elliptical cyst, the pigmented eye-spots being still pre- 

 served, fig. E. The cyst grows and elongates. The 

 body is then covered by an external cuticle, under 

 which is a sparse musculature, followed by an epithe- 

 lium, which lines the cavity, and forms the greater 

 part of the thickness of the body-walls. The author 

 gives some further structural details. Sometimes, 

 but less frequently than in other species, these sporo- 

 cysts multiply by transverse division, effected by a 

 gradually deepening constriction about the middle of 

 the body. 



The next larval forms, the rediae, are developed 

 within the sporocyst. The cells, which each give rise 

 to a redia, are in part soon present in the embryo; but 

 they increase later by proliferation of the cells lining 

 the cavity of the sporocyst. The first clearly recog- 

 nized appearance of the rediae is as a morula-like 

 cluster of cells, which soon assumes the gastrula 

 form. An external membrane appears, and, later, a 

 pharynx. There are several germs in each cyst, 

 usually one redia (less frequently two) nearly ready 

 to leave the sporocyst, with two or three germs of 

 medium size, and several small ones, fig. B. When 

 ready to leave the cyst, the redia, by its own motions, 

 makes a forcible exit by rupturing the walls of the 

 sporocyst. The free rediae force their way through 

 the tissues of the host, and are found especially in 

 the liver. They increase in length to 1..3 mm. or 1.6 

 mm., fig. C; a collar, g, being formed, meanwhile, 

 a little behind the pharynx. In other respects, ex- 

 cept the possession of a digestive tract, the rediae 

 resemble the sporocysts in structure. They are, how- 

 ever, more muscular, and present other differences, 

 which the author describes. There is present a dis- 

 tinct birth-opening at the side of the body, a little 

 behind the collar, which permits the exit of the brood 

 from within the redia. The germs, sp, develop simi- 

 larly to those of the sporocysts, but are more numer- 

 ous. Sometimes they form rediae, and sometimes 

 cercariae; yet the early stages of the spores are the 

 same in either case. A germinal cell, forming part 

 of the internal lining of the posterior end of the body- 

 segments, forms a morula. A gastrula enlarges, and 

 gradually assumes a shape that reveals whether it 

 shall become another redia, or a cercaria. There may 

 be as many as twenty-three spores in various stages 

 of development in one redia. It is probably the 

 temperature which determines whether rediae or cer- 

 cariae are produced; since the former are produced 

 during the warm, the latter during the cold months. 

 The development of the cercariae, the next form 

 in the series, takes place, as we have seen, in the 

 redia. As the oval enterate spore increases in size, 

 it assumes a more elongated shape; whilst one end 

 becomes more attenuated than the other, and finally 

 is constricted off to form the tail. The thicker por- 

 tion becomes the body proper, and in it are developed 

 the bifurcate intestine and other organs. Certain 

 cells, F, later develop into the organs for secreting 

 the cyst; and many of the cells in the body of the cer- 

 caria are crowded with most remarkable rod-shaped 

 bodies, G, closely resembling bacteria in size and 

 shape, reaching a lengtli of 0.006 mm. In an adult 

 redia, with a brood of twenty or so, there will be one, 

 two, or three cercariae approaching complete devel- 

 opment. 



As soon as the cercaria has reached the limit of 

 development within the redia, it escapes from the 

 parent by the birth-opening. When free, D, the cer- 

 caria is very active, and constantly changes its form. 

 Its most striking characteristic is the presence of the 



cystogenous cells, D, c, before mentioned. These are 

 large, and so crowded with coarse, highly refractile 

 granules as to be rendered quite opaque, F. They are 

 arranged in two-lobed masses, extending along each 

 side of the body, and connected together just in front 

 of the ventral sucker. 



By the aid of its suckers, o and s, and tail, the tad- 

 pole-shaped cercaria crawls or wriggles its way out of 

 its host. When the infested snails are kept in an 

 aquarium, the cercariae may occasionally be found 

 swimming about in the water, but not long; for, on 

 coming in contact with the side of the aquarium or 

 the water-plants, it proceeds to encyst itself. The 

 process can be readily observed under the microscope; 

 for, on a glass slide, the cercaria soon comes to rest. 

 It assumes a rounded form; whilst a mucous sub- 

 stance is poured forth all over the body, together 

 with the granules of the cystogenous cells. The tail 

 is shaken off either before or during encystation, 

 whichjis completed in a few minutes. These cysts 

 are the means of infecting the final vertebrate host 

 of the parasites ; the infection being rendered possi- 

 ble by the habits of the intermediate host, Limnaeus 

 truncatulus, which might well be termed amphibi- 

 ous, so strongly is its habit of wandering on land 

 developed. Indeed, they can remain on land for long 

 periods, and resist even prolonged droughts: hence, 

 when in the water, the snails become infested, and, 

 when on land, leave the cercariae that crawl out of 

 their first host scattered over the fields, where they 

 encyst on the grass, and are eaten by the sheep and 

 other animals. 



In the stomach the cyst is dissolved, leaving the 

 worm free. The worm then makes its way into the 

 liver, and probably in about six weeks begins to pro- 

 duce eggs, growing meanwhile. During its growth, 

 its external form changes, the simple forked intestine 

 develops many coeca, the posterior sucker is greatly 

 enlarged, and the sexual organs are matured. There- 

 after, the wondrous cycle of metamorphoses and emi- 

 gration recommences with the new eggs. There are, 

 perhaps, no other instances more striking, of the 

 adaptation of animal species to particular conditions 

 of existence, than we find in histories of such para- 

 sites as the trematode worms, of which we have nar- 

 rated one life-history. Chables S. MiJsroT.. 



FLUORINE MINERALS. 



P. Gkoth has carefully reviewed [Zeitschr. kryst., 

 vii. 457) the following minerals, mostly from Green- 

 land : — 



Pachnolite. — This is shown to be entirely distinct 

 from thomsenolite. The pure crystals were submit- 

 ted to J. Brandl for analysis, who found that they cor- 

 responded closely to the formula Na F . Ca F2 . AIF3. 

 It is distinguished from thomsenolite by its absence 

 of water, and has arisen from the analogous mineral 

 cryolite by the substitution of a calcium atom for 

 two atoms of sodium. Heated in the closed tube, 

 the mineral decrepitates violently, covering the sides 

 of the tube with a white powder. The crystals are 

 monocliiiic. Almost all show the form of slender 

 prisms, the largest from 2 to 3 mm. long, and 0.5 

 mm. thick-, terminated at one end by an apparently 

 rhombic pyramid, and at the other by two basal 

 planes making a very obtuse angle with one another, 

 showing the twin nature of the crystals. The twin- 

 ning plane is parallel to the ortho-pinacoid; and the 

 two halves are so equally developed that the two hemi- 

 pyramids appear above like a very perfect rhombic 

 pyramid. The prismatic faces are finely striated in 



