11 
vesicle; this latter gives off longitudinal branches; these in turn give off secondary 
branches, which ramify through the body, each small branch ending in an excretory 
cell. 
Nervous 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 frecpiently be seen in fresh specimens. 
Life history. — See p. 2.3. 
KEY^< TO THE GENERA OP TREMATODA REPORTED FOR MAN. 
(For species parasitic in man and thus far found in the United States, follow roman 
type. ) 
]. Two suckers (acetabula) present 2 
One sucker (acetabulum) present (Monostomidse) Monostomulum (p. 12) 
2. Ventral acetabulum on ventral surface of anterior half of body 3 
Ventral acetabulum at posterior end of body, terminal or subterminal {Param- 
pliistomidsc) Gastrodiscus (p. 45) 
3. Monecious (hermaphrodites) (Fasciolidse) 4 
Diecious (sexes separated) (Schistosomidfc) Schistosoma (p. 50) 
FASCIOLID.E. 
4. Mature 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 1 . .Again odist omum (j>. 12) 
5. Genital pore not surrounded by a circular, muscular, acetabulum-like struc- 
ture 6 
Genital pore poster o-lateral of acetabulum and surrounded by a circular, muscu- 
lar structure tuhich looks like a third acetabulum Heterophyes (p. 43) 
6. Genital pore between oral sucker and acetabulum 7 
Genital pore postero-median or postero-lateral of acetabulum. (p. 15) 
7. Intestinal ceca dendritic Fa.sciola (p. 21 ) 
Intestinal ceca not dendritic 8 
8. Body large, usucdly over 25 mm. long, and 14 mm. broad; ovary branched; 
vitellogene glands e.vtend to po.sterior end of body, caudad of testicles. 
Fasciolo})sis [\). 30) 
Body usually less than 20 mm. long, and rarely over 5 mm. broad; ovary 
not branched; vitellogene glands do not extend caudad of testicles 9 
9. Testicles anterior of ovary and uterine coils Dicrocoelium (p. 28) 
Testicles posterior of ovary and uterine coils Opisthorchis (p. 30) 
«This key is based primarily upoii.the species found in man, and on this account 
should not be relied upon for other species found in other animals. 
