PEATVRELMINTHES 95 
B. DENDROCOELIDA. — Forms with flattened body and 
branched “intestine. Testes and ovaries follicular. 
Otocysts very rare. 
Tribe 1. Tricladida.— Body elongated. Intestine in three 
main branches, which diverge from a cylindrical retractile 
proboseis, Testes and vitellarium follicular. Germa- 
rium compact. Genital aperture single. 
Planaria and Dendrocoelum live in fresh water. 
Ganda, which shows traces of metameric structure, and 
Bdelloura, which is an ectoparasite of Limulus, are 
marine. The leech-like land planarians are chiefly 
tropical.  Rhynchodemus terrestris and  Geodesmus 
bilineatus are European. The former is found in 
England amongst damp bark, etc., in gardens and 
woods. 
Tribe 2. Polycladida.— Body thin and broad. Numerous 
branched intestinal caeca open into the stomach. Testes 
and ovaries follicular. Genital apertures separate, the 
male anterior to the female. A ventral sucker some- 
times present. Marine. Thysanozoon, Yungia. 
Crass II. CESTODA. 
CHARACTERISTICS. — Hndoparasitic Platyhelminthes, devoid of 
mouth and alimentary canal. Organs of adhesion, in the 
shape of chitinoid hooks or suckers, enable the parasite to 
attach itself to its host. The nervous system is composed of 
a cerebral ganglion and two lateral nerve cords. The coelom 
is represented by irregular splits in the mesodermie paren- 
chyma. The exeretory system ws of the Platyhelminthine 
type, opening to the exterior by one or more pores, which are 
sometimes furnished with a pulsating vesicle. The Cestoda 
are hermaphrodite, the generative organs being usually 
repeated many times. 
One of the commonest Tapeworms found in man is Zaenia 
saginata (mediocanellata) (Fig. 66), and it will be a convenient 
form to describe. 
The worm in question may attain a length of five or six 
yards, and may be composed of many hundred segments, often 
