11 
reside; this latter gives off longitudinal branches; these in turn give off secondary 
branches, which ramify through the body, each small branch ending in an excretorv 
cell. 
Xervous system. — A set of ganglia is found at each side of the pharynx; these ganglia 
are connected by a dorsal commissure and give off numerous nerves to various parts 
of the body. The largest nerves are the two ventral longitudinal nerves which run 
antero-posteriorly, and can frequently be seen in fresh specimens. 
Life history. — See p. 23. 
KEY^< TO THE GENERA OF TREMATODA REPORTED FOR ^fAN. 
(For species parasitic in man and thus far found in the United States, follow roman 
type. ) 
1. Two suckers (acetabula) present 2 
One sucker (acetabulum) present (Monostomidae) Xonostomulum (p. 12) 
2. Ventral acetabulum ou ventral surface of anterior half of body 3 
Ventral acetabulum at posterior end of body, terminal or subterminal {Param- 
jjhistomidee) Gastrodiscus (p. 45) 
3. Monecious (hermaphrodites) (Fasciolidae) 4 
Diecious (sexes separated) (Schistosomidae) Schistosoma (p. 50) 
FASCIOLID^E. 
4. yiature distomes, or worms developed at least to a stage which permits a 
determination of the genus 5 
Immature distomes, or forms not sufficiently developed to permit a deter- 
mination of the genus Agamodistomum (p. 12) 
5. Genital pore not surrounded by a circular, muscular, acetabulum-like struc- 
ture 6 
Genital pore jjostero-lateral of acetabulum and surrounded by a circular, muscu- 
lar structure which looks like a third acetabulum Heteropjhyes (p. 43) 
6. Genital pore between oral sucker and acetabulum 7 
Genital pore postero-median or postero-lateral of acetabulum. .Pa mgOR onus (p. 15) 
7. Intestinal ceca dendritic Fasciola (p. 21) 
Intestinal ceca not dendritic 8 
8. Body large, usually over 25 mm. long, and 14 nim. broad; ovary branched; 
vitellogene glands extend to posterior end of body, caudad of testicles. 
Fasciolopsis {p. 
Body usually less than 20 mm. long, and rarely over 5 mm. broad; ovary 
not branched; vitel4og^»e-giandsd<> not-es4end Ciaudad-Aif testieiee 9 
9. Testicles anterior of ovary and uterine coils Dicrocceliurn (p. 28) 
Testicles posterior of ovary and uterine coils OpAsthorchis (p. 30) 
«This key is based primarily upon the species found in man, and on this account 
should not be relied upon for other species found in other animals. 
