FLAT-WO RMS. 
461 
PILIDIUM LARVA, WITH NEMERTINE WORM 
DEVELOPING INSIDE. 
shorten its body to a very considerable extent. The chief centre of the nervous 
system lies in the head, and from this portion, which may be called the brain, 
nerves pass forwards to supply the suckers, while a single stout cord runs back¬ 
wards on each side to the end of the body, 
lying close to the edge of the segments. As 
already pointed out, there is no trace of a 
mouth nor intestinal canal, although there is 
an excretory organ, consisting of a ring-shaped 
vessel in the head, from which four tubes, 
corresponding in position with the sucker, are 
prolonged backwards. Two of these soon 
vanish, but the others lying near the edges of 
the segments, close to the inner side of the 
nerve-chords and the longitudinal muscular 
band, extend to the hinder end of the body, where 
they unite and communicate with the exterior 
by a common aperture. At the hinder end 
of each of the segments these two ducts are 
united by a third, which runs across from side 
to side. I 11 addition to these structures, each 
fully-grown segment contains a complete set of organs for the formation and 
fertilisation of eggs, of which an immense number are developed. The mature 
segments begin at a distance of about a foot from the head, 
and those at the hinder end of the body are the first to become 
distended with eggs ready to hatch. The eggs, however, are 
not laid within the human body, but the ripe segments break 
off one by one, beginning with the last, and pass out of the 
intestine. The rup¬ 
ture is effected by 
the contraction of 
the muscular fibres, 
which acts upon the 
transverse vessel of 
the excretory system 
of the segment in 
front, in such a way 
that a fresh terminal 
pore is formed. The 
ripe ‘proglottis, or 
segment ready for 
separation, is little 
more than a sac 
that is crammed with 
minute spherical eggs, set free by the bursting of its walls. In this way the 
eo-o-s, which, on account of their thick protecting shell, are able to withstand the 
most unfavourable conditions, are disseminated abroad; and, owing to their vast 
HUMAN TAPE-WORMS. 
a Tcenia soliuvi (nat. size) ; b, c, d, Head enlarged ; e, Segments ; 
f Segment of Tcenia saginatci (enlarged). 
