162 COMPARATIVE ANATOMY CHAP. 



during copulation. This may be developed independently as an acces- 

 sory organ of the female copulatory apparatus. There is in many 

 forms another broad, round, or pear-shaped accessory organ, the 

 reeeptaeulum serainis, a reservoir in which the semen is preserved 

 after copulation. 



In the Trematoda, many Cestoda, and in Trigonoporus among the 

 Polyclada, the ootype, or the uterus, or the egg passage, is connected 

 with the exterior by another special passage, Laurer's canal, already 

 mentioned. The physiological signification of this canal is not yet 

 certainly understood. 



D. The Position and Number of the Copulatory Apparati and 



the Sexual Apertures. 



It may be considered the rule that one male and one female 

 copulatory apparatus are present, and that each opens externally by 

 its own special aperture somewhere in the middle line on the ventral 

 side. The two sexual apertures are generally very near each other, 

 and in many forms most Trematoda, Cestoda, and Triclada, and in many 

 Polyclada and Ilhabdoccelidce come to lie in the base of a more or less 

 deep depression of the outer skin atrium genitale so that only 

 one common outer sexual aperture is present. 



In this point there is great variety in details, and many often striking deviations. 

 In the Polyclada the sexual apertures always lie behind the mouth, in the Cotylca, 

 in particular, between the sucker and the mouth. The male aperture always lies in 

 front of the female. Stylochus and Stylocttoplana have a common external sexual 

 aperture. In Anonymus there are several copulatory apparati and sexual apertures 

 in 2 lateral longitudinal rows. Many Pseudoceridoe possess 2 male copulatory 

 apparati. The female copulatory apparatus and its aperture always remain single. 

 In Stylostomum there is one common external aperture for the pharynx and the 

 penis. 



In the Triclada the common sexual aperture lies behind the mouth, the male 

 copulatory apparatus in front of the female. 



In the Rhabdocaslidce the arrangements are extraordinarily varied. There are 

 sometimes two separate apertures, sometimes an atrium genitale, and thus a common 

 external sexual aperture. Sometimes the male aperture lies in front of the female, 

 and sometimes the reverse is the case. In Prorhynchus the male copulatory apparatus 

 opens in the mouth. 



The genital apertures of the Trematoda, which either enter a common shallow 

 atrium genitale or are very near together, generally lie at the anterior end of the body ; 

 in the Distomidcc, between the mouth and ventral sucker. Less frequently they lie 

 at the posterior end of the body (e.g. Gastcrostomum, Opisthotremd), or asymmetric- 

 ally to the left near the anterior edge of the body (e.g. in Tristomum). 



In the Cestoda there is generally a common external genital pore, or else the 

 genital apertures are very near each other. The poms geuitalis or the two genital 

 apertures of each proglottis either lie at the edge (Tctraphyllidce, Tetrarhynchidce, 

 most of the Tceniadce, Triccnophorus), or on one of the flat surfaces, which is 

 therefore the ventral side (Ligula, Bothriocephaliis, Sckistocephalus, a few Tcenice). 

 In Ampkilina they lie at the posterior end of the body. 



