422 PHYSIOLOGY OF THE INVERTEBRATA, 



attached. This gland secretes a substance which becomes 

 the investment of the ova. The terminal portion of the 

 oviduct passes into the uterus. The vagina is usually a long 

 canal, and at its inner end is tho receptaculum seminis. 

 The small ova are either oval or round. They are formed 

 by thi'ee glands — the ovary, vitellarium, and shell gland.* 



As already stated, the Tape-worms are composed of many 

 segments or proglottides, and at the end of the body the 

 segments become detached. In this condition they retain 

 their vitality for some time. The uterus of the proglottis 

 contains very many ova ; and the embryo (ciliated or 

 hooked) is developed in a similar manner to that of the 

 Trniwioda. 



The Tape-worms live a portion of their life in one animal 

 (cystic condition), and the other portion in an entirely different 

 one (cestoid condition). For instance, the following diagram 

 illustrates the life-history of Tivnia solium, which infests 

 man : — 



Tasnia in man 

 (sexual condition). 



Cystic form in pifj /_ \ Six-hooked or ciliated 



(measly pork). embryo. 



it. TT 



The Tape-worms are rarely met with in both the cystic 

 and cestoid conditions in the same animal ; but the cystic 

 form is found in some creature which serves as prey to the 

 animal in which the cestoid form occurs." 



The Annelida. 



The Myzostomea are small unsegmented worms, parasitic 

 on Comatula (Antcdon). These animals are hermaphrodites. 

 The two oviducts open into the cloaca ; and the male organ 



* The ovary forms the nucleus, the vitellarium the yolk, and the shell 

 gland the external covering of the egg. 



