FIG. 110. DIAGRAMMATIC REPRESENTATION OF THE LIFE-HISTORY OF 

 Tienia soliwn, A COMMON TAPEWORM IN MAN, WITH ITS BLADDER- 

 WORM STAGE IN THE PIG. 



The sexual tapeworm lives in man's intestine, unaffected by the 

 digestive juice, absorbing by its whole surface the digested food of its 

 host. It is attached to the wall of the intestine by a head (HD), about 

 the size of a pin's head ; on this head there are four suckers which have 

 to do with adhesion only (there is no mouth nor food-canal), and there 

 is also a circle of minute gripping hooks. Behind the head there is a 

 growing area or neck (,V) where joints or proglottids (PR) are always 

 being formed. The chain of joints, which change in shape as they grow 

 older and are pushed further from the head, may be several yards long 

 and is like a piece of white tape. Each joint has a complete set of herma- 

 phrodite reproductive organs, and self-fertilisation (autogamy) occurs. 



The ripe joints at the end of the tape-like chain (often called a strobila) 

 contain only a branched uterus (ill) full of enshelled developing eggs. 

 The genital aperture (f/a) is seen at the side, but the eggs are eventually 

 liberated by the bursting of the proglottis and uterus. This happens 

 when the last proglottis is separated off and passes out with the faecal 

 matter. The microscopic egg-shells may be carried by runlets of water 

 or by wind ; they are very resistant. The life-history is not continued 

 unless a pig swallow the microscopic enshelled embryo. 



In the pig's stomach the shell is dissolved and a six-hooked (hexacanth) 

 embryo is liberated. It bores its way to the muscles (m) and there settles 

 down. It loses its hooks, increases in size, and becomes a passive, vege- 

 tative, asexual bladderworm or proscolex (Psc). An ingrowing bud 

 from the wall of the bladder forms the future " head," or scolex (Sc). 

 This is afterwards everted, and then the bladderworm consists of a small 

 head attached by a short neck to a bladder (BL) about the size of a small 

 pea. If man unwittingly eats the imperfectly cooked "measly" pork, 

 the head fixes itself to the wall of his intestine and buds off a chain of 

 joints. The chances are happily several millions to one against an en- 

 shelled embryo becoming a tapeworm. 



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