CESTODA. 191 
nervous system consists of two or more longitudinal nerve- 
strands and anterior commissures ; there are no special sense 
organs. There ts no alimentary system; the parasite 
Jtoating 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 deferehs, 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 
jinal 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 Tenia 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 froglottides 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- 
