ARRANGEMENT OF THE GENITAL OPENINGS. 279 



position of the generative organs e.g., of the testes dorsally affords ns 

 certain fixed points for their determination. x 



As we shall afterwards see, there are also Trematodes 

 with marginal genital openings, but their number is so small 

 that this position of the openings must be regarded only as ex- 

 ceptional, while in the case of the Cestodes it is almost the 

 general rule even outside the group of the Teeniadee. On a priori 

 grounds one would expect these peripheral openings to occur 

 on either side, but there are only few species 

 in which this is the case (Tcenia elliptica, 

 Tcenia expansa, &c., Fig. 143). In spite of 

 this, it must be regarded as forming the basis 

 of the common asymmetrical formation, and 

 this is proved by certain individual variations, 

 which we shall have further occasion to dis- 

 cuss particularly. Thus one finds, for example, 

 joints of Tcenia solium with symmetrical 

 marginal pores, and joints of T. elliptica occa- 

 sionally asymmetrical. In bilateral animals, 

 the unpaired mesial organs are, as is well 

 known, very often represented by two lateral 

 symmetrical organs. If further proof be FIG. 143. Sexually mature 

 needed of the morphological identity of the proglottis of T. elliptica. 

 lateral organs in Tcenia with the median structures of Botliriocephalus, 

 this might be noted, that in some species of Botliriocephalus (B. 

 fasciatus, B. tetrapterus), the genital openings lie, not, indeed, ex- 

 actly marginally, but regularly, right and left of the middle line. 

 And while this occurs here as a rule, it also appears as an exception 

 in other species. 



The Anatomy of Cestodes. 



Sommer and Landois, " Ueber den Bau der geschlechtsreifen Glieder von Bothrio- 

 cephalus," Zeitschr. f. iviss. Zool., Bd. xxii., pp. 40-100, 1872. 



Schiefferdecker, " Beitrage zur Kenntniss des feinern Baues der Tanien," Jenaische 

 Zeitschr., Bd. viii., pp. 459-488, 1874. 



Steudener, " Untersuchungen iiber den feinern Bau der Cestoden," Abhandl. d. 

 naturf. Gcsellsch. zu Halle, Bd. xiii., pp. 277-366, 1877. 



Kahane, "Taenia perfoliata, als Beitrag zur Anatomic und Histologie der Cestoden," 

 Zeitschr. f. wiss. Zool., Bd. xxxiv., p. 175, 1880. 



1 In the first edition of this work I called attention to the peculiar distribution of the 

 generative organs of Ta>nia>, and indeed of Cestodes generally. My conclusions have since 

 been corroborated, especially by Kahane (Zeitschr. f. wiss. Zool., Bd. xxxiv., p. 214, 1880), 

 and by Moniez (Bullet, scientif. dfp. du Noi'd, p. 220, 1878), who has, indeed, failed to 

 notice my previous observations on the subject. 



