160 COMPARATIVE ANATOMY CHAP. 



2 yolk ducts, which, as well as 2 oviducts, enter the ootype by a 

 common terminal piece, into which the efferent ducts of the shell glands 

 open. From the ootype a canal proceeds to the copulatory apparatus 

 (ov) on one side, and on the other side arises a widened uterus filled 

 with eggs (Fig. 115, u; Fig. 116) running in coils, or provided with 

 lateral sacs ; this uterus often reaches the exterior by a special aper- 

 ture which recalls Laurer's canal in the Trematoda. Where there is 

 only one vitellarium, only one yolk duct naturally enters the ootype 

 (as in Fig. 115). 



II. Male duets. In the Polydada numerous very fine canalicules 

 enter the larger semen duets, vasa deferentia (Fig. Ill, vd), in 

 which the spermatozoa collect, and these canals again have their exit 

 in the male copulatory apparatus (jy). The fine canals correspond to 

 the oviducts, the wider ones to the uterus of the female sexual appar- 

 atus. In the Tridada there are 2 lateral vasa deferentia (Fig. 112, 

 vd), into which some at least of the testes empty their contents 

 direct, while the manner in which the testes which are at a greater 

 distance from the vasa deferentia empty themselves is not yet fully 

 understood. 



In the Rhabdoccela (Fig. 113) the 2 testes are often continued 

 without any sharp demarcation into 2 semen ducts which enter the 

 male copulatory apparatus either separately or by means of a common 

 terminal portion. In the Acoela and most Allowccda special ducts 

 are wanting ; the spermatozoa reach the copulatory apparatus through 

 the parenchyma. Only in the Monotidce among the Alloioccda the 

 transmission takes place by means of special ciliated vasa deferentia. 

 The two testes of the Trematoda (Fig. 114) send out two semen 

 ducts (cd) which unite into one common duct. In the Cestoda also 

 (Fig. 115) many of the numerous canalicules proceeding from the 

 testicle vesicle enter a common vas deferens leading to the male copu- 

 latory apparatus. 



C. The Copulatory Apparatus. 



There is wonderful variety in the structure and position of the 

 copulatory apparatus in the Platodes. Nearly related species often 

 differ greatly in this point. 



I. The male eopulatory apparatus is always more complicated in 

 structure than the female. It consists in the simplest cases of a mus- 

 cular pouch which projects from the surface into the parenchyma, 

 and into whose blind end, which is directed inwards, the semen duct, 

 or ducts, enter. It is found in this form in certain Rhabdocc&lidce. . In 

 most Turbellaria, however, it becomes complicated, and then we can 

 generally distinguish the following distinct portions : ( 1 ) a penis 

 sheath or penis pouch, (2) the actual penis, (3) a seminal vesicle, 

 and (4) a granular gland. The penis and penis sheath show a struc- 

 ture which is on the whole like the structure of the pharyngeal appa- 



