180 



COMPARATIVE ANATOMY. 



either extraordinarily long, or short and simple, or provided with 

 diverticula. These cavities are known as uteri ; for in them the egg 

 is not only enclosed in its shell, but as a rule passes through the 

 early stages of embryonic development. A diverticulum of the 

 female excretory duct, which has generally the form of a stalked 

 vesicle, receives the sperm during copulation. In some cases there 

 is a second diverticulum also ; it serves apparently for the reception 

 of the male organ (Bursa copulatrix). 



The most important complications of this system are seen in the 

 parasitic Platyhelminthes. The preservation of the species is here 

 subject to innumerable difficulties, owing to the animal living in 

 different hosts at different stages of development, and to the 

 wanderings which this mode of life entails ; consequently a large 

 number of ova have to be produced, and the certainty of fecundation 

 insured. 



148. 



The more special characters of this generative system exhibit 



extraordinary variation. The male portion 

 has, in most Turbellaria rhabdoccela, the 

 form of two elongated testicular tubes, from 

 each of which a vas deferens is given off 

 (Fig. 85, t). In the Trematoda, also, the 

 testicles are, as a rule, but few, and rounded 

 or lobate ; these are represented in the Tur- 

 bellaria dendrococla, as well as in several 

 rhabdoccela (Macrostoma), and Cestoda by 

 a number, and often a very large number, 

 of small follicles, scattered in the parenchyma 

 of the body (Fig. 86, t) ; these are connected 

 together by long efferent ducts. They may 

 form a single row on either side (land Pla- 

 narians). The excretory ducts either form a 

 common vas deferens, or each passes sepa- 

 rately to a terminal portion, which is con- 

 tinued into the copulatory organ. The 

 common excretory duct forms the seminal 

 vesicle, or, as happens in a few cases, it is 

 formed by enlargements of the separate vasa 

 deferentia. The copulatory organ (Fig. 85, 

 p ; Fig. 87, p 1 ) is generally large and mus- 

 cular, and the seminal vesicle often appears 

 to be an appendage of it. It lies in a special 

 cavity leading to the genital pore (penial 



Fig. 85. 



system 



viridis. 



Vasa deferentia 



minal vesicle, 



Generative 



of Vortex 



1 1 Testes, vol 



vs Se- 



p Pro- 



tractile organ of copula- 

 tion, oo Ovaries. gu 

 Yolk-glands. rs Eecep- 

 fcaculum seminis. v Va- 

 gina, u Uterus (after 

 M. Schultze). 



sheath in Planaria, bag of the 



Cestoda [Fig. 80, cZ] and Trematoda) ; glands 



are sometimes connected with it (Planaria). 



The copulatory organ is, as a rule, protractile, or can be everted, 



whereupon certain spines or hooks which lie on its inner surface, 



