v PHYLUM PLATYHELMINTHES 255 



part of this passage, into which (usually) the shell glands and 

 the ducts of the vitelline glands open, is termed uterus or ootype, 

 while a terminal portion leading from the uterus towards the- 

 exterior is known as the vagina. The uterus receives the ova 

 with their investing yolk matter, and retains them frequently 

 for some considerable time while the shell is being formed and 

 the earlier stages of development are proceeding : often the uterus ; 

 is found to contain large numbers of ova, each enclosed in its- 

 chitinoid shell, in other instances it may enclose only one egg 

 at a time. The vagina is sometimes a copulatory apparatus for 

 receiving the penis, and is often provided internally with horny 

 spines or teeth; sometimes a special sac or bursa copulatrix, lined 

 with spines, acts as the female copulatory organ. A sac opening 

 into the distal part of the oviduct, or that most remote from, 

 the genital opening, is termed receptaculum seminis or receptaculum 

 vitelli (Fig. 190, r.v.) according as it serves as a reservoir for the 1 

 semen received in copulation or for the vitelline matter or yolk.. 

 Male and female ducts sometimes have separate and independent 

 openings ; but very commonly there is a common chamber or 

 genital cloaca into which both lead, opening on the exterior by 

 a single aperture. 



In the Polyclad Turbellaria (Fig. 201) the testes are numerous,, 

 and there are a corresponding number of fine tubes which 

 combine to form the two vasa deferentia leading to the male 

 aperture with its penis. The ovaries consist of numerous small 

 rounded masses of cells, and there are no separate yolk-glands. 

 Numerous narrow oviducts lead from the ovaries, and unite to 

 form larger ducts ; these, in turn, open into elongated uteri, in 

 which numerous eggs collect. The uteri open into a median 

 vagina, into which the ducts of the shell glands open, and in which 

 the eggs receive their chitinoid investment. 



In the Tricladida (Fig. 202) there are also numerous testes, but, 

 the fine tubes connecting them with the two vasa deferentia are 

 absent. There are only two ovaries, situated far forwards, but 

 numerous yolk glands. Two oviducts, into which the yolk is dis- 

 charged from the yolk glands by a series of lateral apertures, lead 

 from the ovaries to unite in a median chamber or uterus receiving 

 the ducts of the shell glands. 



In the Rhabdocceles (Figs. 200 and 204) there are usually only 

 two testes and two vasa deferentia leading to the unpaired male 

 aperture at the extremity of the cirrus. The prostate or granule 

 glands a set of unicellular glands, which secrete round bright 

 granules destined to mix with the sperms are specially well 

 developed in the Rhabdocceles, though present in some other 

 Turbellaria, and in certain Trematodes. Ovaries alone are present 

 in some, ovaries and yolk glands in others ; there are either two* 

 ovaries or one only. 



