120 ZOOLOGY. 
in the young, and there are no suckers or hooks; while 
there is but a single set of male and female reproductive 
organs situated in the posterior end of the body, which can 
be detached from the ante- 
y rior part of the body, form- 
s ing a proglottis. In fact, 
LY a this form is a connecting 
e 6 link between the Trematoda 
Fig. 81.—Head of 7. cenurus seen from and Cestodes. Cary op hy aus 
above, with circle of hooks ; a—e, hooks; yn utabilis Rudolphi lives in 
all much enlarged .—After Siebold. i i ; Bs tae 
the intestines of Cyprinoid 
fishes ; the young in a worm, Tubifex rivulorum. 
Tetraryhnchus is provided with four very long slender 
extensile spiny cephalic processes or beaks. The young live 
encysted in bony fishes, the adults occurring in the intestines 
of sharks and rays. 
In Ligula the body is ribbon-shaped, not jointed, with a 
series of sexual organs, and there are no suckers, and some- 
times no hooks. JL. simplicissima Rud. lives in fishes and 
amphibians, and attain maturity in the intestines of water- 
birds, which feed on the former animals. This genus con 
nects the simpler tape-worms with Bothriocephalus and 
Tenia. 
Crass —PLATYHELMINTHES. 
More or less flattened worms, with the body usually unsegmented ; the 
head in the Cestodes often armed with hooks or suckers. Simple or branched 
(Turbellaria) or forked (Trematoda) digestive tract, but no general body- 
cavity. (The digestive cavity is entirely wanting in the Cestodes.) Nervous 
system represented by a double cephalic ganglion, with two or more nervous 
cords. A system of vessels corresponding to the water-vuscular system of 
Echinoderms, but supposed to be mainly excretory in function. Mone- 
cious, rarely bi-secual. Ovaries differentiated into a germigene and vitel.. 
logene ; often parthenogenetic, accompanid by strobilation in the tape- 
aorms. When alternation of generations occurs by budding, the sexual ani- 
mals are united with their nurse or a sexual form into a polymorphic colony. 
Order 1, Turbellaria.—Flattened ovate worms, with a nervous gan- 
glion in the head ; usually eye-specks ; body externally cili- 
ated, with a much-branched digestive canal. Nettling 
organs often present. Bisexual, rarely unisexual; strobi- 
