CESTODA. 191 



nervous system consists of two or more longitudinal nerve- 

 strands and anterior commissures ; there are no special sense 

 organs. There is no alimentary system ; the parasite 

 floating in the digested food of its host absorbs soluble 

 material by its general surface. There is no vascular nor 

 respiratory system, and a body cavity is represented merely by 

 irregular spaces in the solid parenchymatous tissue. In some 

 of these spaces there are "flame-cells" which Lie at the ends of 

 the fine branches of longitudinal excretory tubes, which are 

 united in a ring in the head, are connected transversely at 

 each joint, and open terminally by one or more pores. All 

 tape-worms are hermaphrodite, and most, if not all, are 

 probably self-fertilising. The male reproductive organs in- 

 clude diffuse testes, a vas deferens, and a protrusible terminal 

 cirrus. The female organs include a pair of ovaries, yolk 

 glands, a shell gland, a vagina by which spermatozoa enter, a 

 receptacle for storing spermatozoa, and a uterus in which the 

 ova develop. The embryo develops within another host into a 

 proscolex or bladder-worm stage, which forms a " head " or 

 scolex. When the host of the bladder-worm is eaten by the 

 final host, the scolex develops into an adult sexual tape-worm. 

 With the conditions of endoparasitic life may be associated the 

 occurrence of fixing organs, the absence of sense organs, the low 

 though somewhat complex character of the nervous system, the 

 entire absence of a food canal, and the prolific reproduction. 



Life history of Tsenia solium. This is one of the most 

 frequent of the tape-worms infesting man. In its adult state 

 it is often many feet in length, and is attached by its "head" 

 to the wall of the intestine. The head bears four suckers 

 and a crown of hooks, and buds off a long chain of joints, 

 which develop complex reproductive organs as they get 

 shunted farther and farther from the head. The last of the 

 joints or proglottides is liberated (singly or along with 

 others), and passes down the intestine of its host to the 

 exterior. It has some power of muscular contraction and 

 of movement, and it is distended with little embryos within 

 firm egg-shells. When the proglottis ruptures, these are 

 set free. 



In certain circumstances, the embryos, within their firmly 

 resistant egg-shells, may be swallowed by the omnivorous 

 pig. Within its alimentary canal the egg-shells are dis- 



