EXAMINATION OP HELMINTHES 



129 



t.r.c.d. 



parency they are readily examined in the fresh state and, if carefully 

 preserved and coloured, show the details of their structure with great 

 clearness. Under the new sys- 

 tem of classification, moreover, 

 they are eminently suitable sub- 

 jects with which to begin the 

 study of the endoparasitic Tre- 

 matodes. 1 Should these varie- 

 ties be unobtainable, the small 

 transparent Trematodes found 

 in the intestine of frogs, and 

 which are easily come by, should 

 be used in their place. Other 

 species inhabit the frog, which, 

 though useful, are rather more 

 difficult subjects for experiment. 

 They are found in the oral 

 cavity, generally under the 

 tongue; in the lungs, where they 

 live in company with a Nema- 

 tode, Rhabdonema nigroveno- 

 sum ; and in the urinary bladder, 

 where the monogenetic Trema- 

 to&e t Polystomumintegerrimum t 2 

 is also to be found (fig. 51). 



After their removal from the 

 frog, the Trematodes should be 

 put into some indifferent fluid 

 upon a glass slide, and covered 

 with a cover-glass of sufficient 

 weight to exercise a slight pres- 

 sure upon the objects. The 

 movements of the parasites at 

 first make them difficult of ob- 

 servation, but after an hour or 

 so these will cease and the struc- 

 tural details of the organs 

 may be studied. The excretory 



1 The digenetic Trematodes were for a long time arranged under three headings, 

 Monostomum, Distomum, and Amphistomum, classification being based upon the 

 number and position of the suckers. These three genera are now regarded as 

 families, and are subdivided into numerous small classes, the basis of classi- 

 fication, in this instance, being the arrangement and structure of the reproductive 

 organs. 



2 The oral cavity harbours Distomum ovocaudatum, now Halipegus ovocaudatus ; 

 three varieties are found in the lun^s, Distomum cylindraceum, now classed with 



9 



d. 



h. 



FIG. 51. Polystomum integerrimum. Mag- 

 nified, d., Intestine. 7i. f Large, hk., small, 

 hooks of the sucking disc, l.c.d., Openings of 

 the yolk-glands (not shown), m., Mouth. Oot., 

 ootype. ov., Ovary, su., Suckers of the attach- 

 ment disc, t.r.c.d., Transverse yolk-duct. Ut., 

 Uterus containing eggs. v., Mouth of the 

 vagina, v.d.e., Vas deferens. v.d.i., Ductus 

 vitello-intestinalis. (After Zeller.) 



