NO. 1105. PROCEEDINGS OF THE NATIONAL MUSEUM. 205 



studied further when proper material is found. It is, however, by no 

 means the only case of homoplasy brought forward in helminthology. 

 I have 1 elsewhere called attention to the branching of the intestine 

 in two distinct types of flukes, as represented by Fasciola liepatica and 

 Distomum delpliini. The presence of more than two testicles, as in 

 two or more other distinct types of flukes represented by Pleorchis 

 polyorcUis and P. mollis on the one hand and Distomum cycjnoides or Z>. 

 richarcU on the other is, in iny opinion, another case of homoplasy. 

 The development of double-pored forms in two distinct families (the 

 Bothriocephalkhe Blanchard's genus Krabbea, and a number of cases 

 in which a few double-pored segments have been found in Bothrio- 

 cephalm latm and the Taeniidue forms a fourth case of the same nature. 



One of the greatest criticisms upon the present classification of para- 

 sites seems to rne the fact that systematists have not taken into con- 

 sideration the principle so well established in paleontology and other 

 branches of zoology, namely, that the same structure may develop 

 independently in several parallel series of animals. The sooner this prin- 

 ciple is acknowledged the sooner we shall have a natural classification. 



Zone of lateral growth. A comparison of the older and younger seg- 

 ments of numerous Ta3uiida3 shows that as the segments grow broader, 

 the median field namely, the space between the two ventral canals 

 increases much more rapidly in proportion than the lateral fields 

 namely, the space between the ventral canals and the lateral mar- 

 gin. This establishes the median field as the greatest zone of lateral 

 growth. No particular narrow zone of the median field can be looked 

 upon as the zone of lateral growth for all cestodes, as is shown by a 

 comparison of a large number of forms. In Tccnia (type T. solium], 

 for instance, the entire median field appears to participate in a more or 

 less uniform lateral growth, for the genital glands retain their rela- 

 tive position, increasing in size in proportion to the increase in size 

 of the segment; the same appears to hold for Davainea. In the 

 anoplocephaline forms we find some interesting variations in the zone 

 of growth, as is shown by the relative position of the ovaries to the 

 ventral canals or to the median line. In Thysanosoma yiardi the ovary 

 bears an almost constant relation to the ventral canal, while the dis- 

 tance between the ovary and the median line constantly increases as the 

 segment grows broader; in this case, therefore, there is but little growth 

 between the ovary and the canal, while the growth in the median side 

 of the ovary is very marked. The same holds true to a lesser degree 

 in Bertia americana. In Moniezia expansa and M. pianissimo, also the 

 relation of the ovaries to the ventral canals remains almost constant, 

 while the chief zone of growth is in the median field between the 

 ovaries. 



'The anatomy of the large American fluke (Fasciola magna] and a comparison 

 with other species of the genus Fasciola s. st. vide p. 221, Jour. Comp. Med, Vet. 

 Arch., 1895. 



