GENEEATIVE OEGANS OF VERMES. 179 



meiit is not unfrequently connected with, great coinplicatious in 

 comparison with which the arrangements seen in dioecious Vermes 

 are very simple. 



The simplest state is seen in the Bryozoa., the generative pro- 

 ducts of which are developed either on tlie inner face of the body- 

 wall from simple aggregations of cells^ which give rise to seminal 

 elements or to ova ; or they arise on a chord which extends from the 

 enteric canal to the inner wall of the body (funiculus) (Fig. 71^ n). 

 The mature generative products pass into the coelom, and are 

 thence passed out into the surrounding water by the orifice of 

 communication mentioned above. The two sexes are ordinarily 

 united in the same individual, the male and female germinal glands 

 being separate. 



In all phylactoloematous fresh-water Bryozoa, special bodies, 

 (statoblasts) foi'med of an aggregation of cells, are developed in the 

 body- wall, at the points where the ova are formed ; these break off, 

 just hke the ova, and form free-living buds. Various differentiations 

 give rise to complicated shell-structures around them. 



§ 147. 



Hermaphroditism obtains also in the Platyhelmiuthes generally 

 (Turbellaria, Trematoda, Cestoda). The two groups of sexual 

 organs are, as a rule, united at a common orifice, being other- 

 wise separated from one another, and embedded in the paren- 

 chyma of the body. The secreting glands (testis and ovary) are 

 generally small and simple in character. The excretory duct 

 and the glandular organs connected with them, as well as the 

 diverticula, or pouch-like appendages of the former, which act as 

 places for the development of the fertilised ovum, or as receptacles 

 for the semen, take by far the greatest share in the complication of 

 the apparatus. 



As to the male organs, the testes vary in number, and are 

 generally indistinctly marked-off spots for the formation of the 

 semen, which reaches the common duct by narrow seminal ducts. 

 A widened portion of the former has the function of a seminal 

 vesicle, and its end is converted into a protractile organ, which 

 serves as a penis. 



The ovary forms the most important part of the female organs. 

 An organ, generally widely branched, is connected with its ducts — the 

 yelk -gland: in the lobules of this gland cells are produced. The cells 

 of this gland are used in building up the embryo, a number of them 

 together with the egg-cell forming the egg. The origin of the yelk- 

 gland is probably to be found in the division of labour of a primi- 

 tively very large ovary, a portion only of which has contiiiued as 

 ovary, while the cells of the other parts have ceased to be ovarian 

 germs, but becoming surrounded by the products of the fission of 

 the egg-cells, are taken into the future body of the embryo. The 

 oviducts and the ducts of the yelk-gland unite in a canal of varying 

 length, which, according to the number of eggs to be developed, is 



N 2 



