118 ZOOLOGY. 
This worm is extremely rare in America, but is common in 
Western Switzerland and Central Europe, and in the north-. 
western and northern provinces of Russia, Sweden, and 
Poland. It is sometimes twenty-five feet long, and nearly 
an inch broad, with 4000 joints. The club-shaped head is. 
unarmed, and the first sexually mature segment is about 
Fig. 19.—Male reproductive organs, with parts of the female of Bothriocephalus: 
Zatus. t, testicular follicles, only a part are represented ; ve. their excretory ducts; 
vd, vas deferens 3 ¢, cirrus ; eb, sac Containing the cirrus; u, uterus containing eggs: 
ov, ovary ; gi, shell-gland ; e, water-vascular trunks ; v, vaginal canal.—-After Landois 
and Sommer ; from Gegenbaur. 
the 600th from the head. Leuckart has suggested that. 
the young of this tape-worm originate in salmon and. 
trout. 
The sheep-hydatid is the larva of Tenia cenurus (Figs. 
80 and 81), the adult infesting the dog. The presence of 
one or several of the hydatids in the brain of the sheep pro- 
duces the ‘‘ staggers ’’ or vertigo. The vesicle varies in size- 
from a pea to a pigeon’s egg. It is bladder-like, filled with. 
a clear pale yellow albuminous secretion, with a great num- 
ber of retractile papille (D, g), which are the tape-worm heads: 
connected by narrow stalks to the common vesicles support- 
