in PLATODES SEXUAL ORGANS 161 



ratus described above. The penis is, in fact, a muscular circular fold 

 which projects into the penis sheath from its wall in a manner similar 

 to that in which the pharynx projects into the pharyngeal pouch. As 

 the pharynx is protruded out of the pharyngeal pouch through the 

 mouth, so is the penis protruded through the sexual aperture. During 

 copulation the wall of the penis sheath also is pushed out or evagin- 

 ated. The penis sheath is occasionally double, or there are several 

 sheaths, each of which is related to the one outside it as the penis is 

 to the penis sheath, and the whole apparatus may be telescopically 

 extended and protruded. The penis is sometimes conical, some- 

 times cylindrical, sometimes bent, either naked or armed in various 

 ways. Its free end is often a hard chitinous tube. Between the 

 penis on the one side and the terminal portion of the semen duct 

 on the other, there is a vesicular expansion with muscular wall, the 

 seminal vesicle (Fig. Ill, sb), in which the semen collects, and 

 which, by its contraction during copulation, causes the ejection of 

 the semen through the penis canal (ductus ejaculatorius). In nearly 

 all Turbellaria there is, in connection with the male copulatory 

 apparatus, a granular gland, the structure of which differs greatly 

 in details. It forms a finely granular secretion, which mixes with the 

 semen. 



The male copulatory apparatus of the Trematoda (Fig. 114, cb) and 

 that of the Cestoda (Fig. 115, cb) are very similar in structure. 



In mechanism it corresponds with a Tetrarhynchus proboscis. 

 There is a cylindrical or club-shaped penis sheath. Into the inner 

 blind end of this penis sheath enters the unpaired terminal portion of 

 the vas deferens. On entering the penis sheath it generally expands 

 into a seminal vesicle, and then runs as a coiled thin tube through the 

 penis sheath to emerge at the sheath's outer end through the male 

 genital aperture. This tube, which is often furnished internally with 

 barbed hooks or covered with an elastic cuticle, is forced out as an 

 actual penis by the contraction of the penis sheath. The space 

 between the penis and the penis sheath is filled with loose connective 

 tissue. The penis and penis sheath are generally called cirrus and 

 cirrus pouch in the Trematoda and Cestoda. Glands connected with 

 the copulatory apparatus have also been observed. 



II. The female eopulatory apparatus very often, in many Tur- 

 bellaria and in all Trematoda and Cestoda, consists of a simple tube of 

 varying length, the vagina, which connects the egg passage or the 

 ootype with the female sexual apparatus. This tube often serves 

 merely as a place for depositing the eggs, not for copulation, i.e. it 

 does not receive the penis. This is at least often the case with those 

 Polydada which have more than one copulatory apparatus, but only 

 one female genital aperture. 



In very many Turbellaria, however, the vagina is differentiated 

 into a strong muscular organ, often provided with a hard cuticle, the 

 bursa eopulatrix, which is adapted for the reception of the penis 

 VOL. i M 



