PORUS GENITALIS AND CLOACA. 309 



As has already been mentioned, the male and female organs are 

 differently situated with respect to the two surfaces of the tape- worm, 

 the former belonging to the back and the other to the ventral surface. 

 Here, too, as in the position of the sexual openings, numerous and 

 striking differences appear, which at first sight are little in accord- 

 ance with the general nature of bilateral structure. This is particu- 

 larly true of the forms which have genital openings on one margin, 

 which is the case indeed in the majority of Cestodes, and especially 

 of Tetrabotkria. 



What was formerly said of the genital openings in the Cestodes 

 was only true of the vaginal opening, which is the only female open- 

 ing constantly present in these animals, and is always situated near 

 the male one. 1 These two openings, which alone form what is 

 usually called the poms genitalis of the Cestodes, vary in their 

 position, but the uterine opening, when present, is always found on the 

 ventral surface, and, with the exception of a few species with double 

 uterus and double opening, lies on or near the middle line. 2 When 

 the other genital openings are also veiitrally situated, the uterine open- 

 ing lies a short distance behind them, or more rarely near them. 



There are thus two openings in the porus genitalis of the Cestodes, 

 the male opening and the female vaginal opening. They lie close 

 together, and the male one is generally above. 3 The latter only 

 assumes a lateral situation in very short-jointed tape-worms, such as 

 Tcenia nana and T. perfoliata, and also in Ligula and Sckistocepkalus ; 

 and it is usually so associated with the female, that they have a short 

 and narrow pouch in common, a general cloaca, which is clad with 

 a thin prolongation of the investing cuticle, and, in spite of its origi- 

 nally small size, seems capable of considerable extension. In many 

 species, and especially in the larger ones, a more or less marked 

 swelling surrounds the porus, which is sometimes flat and plate-like, 

 but often deeper, and then protrudes like a papilla on the external 

 surface (Tcenia saginata, &c.). 



From the male opening there generally protrudes a longer or 

 shorter thread-like process, which is known by the name of " cirrhus." 



1 The statement of von Siebold (Vergl. Anat., p. 147), that in Tricenophorus and Tcenia 

 ocellata these openings were situated far from each other, since the vulva was found on 

 the ventral surface, but the penis on the margin, probably rests upon an error. There is, 

 indeed, no doubt that he has made a mistake in the case of Tricenophorus at least, for the 

 vagina is overlooked, and the uterine opening interpreted as a vulva. 



2 It is so in Tricenophorus, whose uterine opening, according to Steudener (Abhandl. 

 naturf. GescUsch. Halle, Bd. xiii., p. 302, 1877), diverges a little towards the opposite side 

 of the peripheral porus genitalis. It is the same in the Ligulidae (see Kiessling). 



8 In Tetrabothrium, van Beneden (loc. cit. ) describes the vaginal as above the cirrhus 

 pouch, but this is a mistake, as the case of Tetrarhynchus has convinced me. 



