Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 333 



pore, which in successive metameres is alternately on the one 

 side and the other (see E.). Into the genital cloaca there also 

 opens a vagina (vag.), which expands to form a receptaculum seminis 

 (r. s.). Closely connected with this are the apertures of four 

 structures : (1) the ovary (ovy.)-, (2) the large uterus (ut.); (3) the 

 yolk gland (vitellarium) (n.) and (4) the small shell gland (sh. gl.). 



The ova passing from the ovary are impregnated by sperma- 

 tozoa in the receptaculum. They also receive yolk (or albumen ]) 

 from the yolk gland, and are encased in a shell from the shell 

 gland. Then they pass into the uterus. As the uterus becomes 

 distended with eggs it assumes the form shown in Fig. 98, H. 

 (ut.), which represents a metamere in the stage figured at F. 

 The other reproductive organs are absorbed or completely 

 masked. When these ripe proglottides break away and are 

 expelled with the excreta, they die and decay; but the eggs 

 which they contain in vast numbers do not readily lose their 

 vitality, and only await favourable conditions for development. 



Before showing what these conditions are, it will be well to 

 suggest the question whether the strobila is one organism or 

 many. Is the scolex an asexual zooid which gives rise by bud- 

 ding to a great number of sexual zooids ? or have we one 

 segmented organism, of which the anterior portion is specially 

 differentiated while all the others are serially homologous, and 

 each provided with male and female reproductive organs ? The 

 former view is that generally held. In which case we have here 

 an instance of alternation of generations in which the proglot- 

 tides are to be regarded as provisional embryonic organisms. 

 On the other view we have provisional embryonic body-segments, 

 a state of things curiously intermediate between embryonic 

 organs and embryonic organisms. 



Within the uterus the ova undergo segmentation with the 

 formation of a morula provided with six stylets (Fig. 98, A.), and 

 enclosed in a shell. If now a portion of a proglottis, or some of 

 the embryos, be swallowed by a pig, the egg-shell is dissolved by 

 the gastric juice of this intermediate host, and further development 

 is rendered possible. The embryo, by means of the stylets with 

 which it is provided, penetrates the tissues of the intermediate 



