244 
thirds of the body of N.agilis tapers, while only the tip of the body of 
N. crassus is conical. 
The body of N.crassus is much more robust than that of JN. agilis. 
This appearance is due primarily to the greater thickness of the body- 
wall in crassus. In the region of maximum diameter of the body the wall 
of N. crassus rarely measures less than 80 p, and is frequently 100 p» thick, 
while in N. agilis the body wall in the same region rarely reaches a thick- 
ness greater than 40 p. This same difference may be expressed in the 
ratio between the thickness of the body-wall and the diameter of the 
body-cavity. In N.crassus the maximum diameter of the body-cavity is 
not more than eight times the thickness of the body-wall, while it is 
usually only about five times the thickness of the wall. In specimens of 
N.agilis studied by the writer the maximum diameter of the body-cavity 
is frequently eighteen or twenty times the thickness of the body-wall: 
The proboscis of N.crassus is conspicuously larger than that of 
N. agilis. 
The male reproductive organs in N. agilis are usually located farther 
from the posterior tip of the body than in N. crassus, and therefore the 
ducts leading from the cement gland and from the testes are longer in 
the former than in the latter. : 
The cotypes upon which the description of N.crassus is based were 
collected by Dr. George R. La Rue from the intestine of the common 
sucker, at Douglas Lake, Michigan, July 20, 1912. 
OCTOSPINIFER, n. gen. 
Generic Diagnosis ——Proboscis short, globose, usually slightly broader 
than long; provided with three circles of eight hooks each. Hooks of 
terminal circle not much larger or stronger than hooks of middle circle 
and but little longer than the root process. Testes elliptical, in contact 
with each other but not joined by a broad contact-surface. Cement gland 
not in direct contact with posterior testis. The two lemnisci dissimilar in 
nuclear content, one possessing two giant nuclei and the other a single 
one. Central nervous-system located at one side of the proboscis- 
receptacle, near its base. 
Type species, Octospinifer macilentus. 
OCTOSPINIFER MACILENTUS, fl. Sp. 
(Pl. XXVI, Fig. 26, 27, 29) 
Body long, approximately cylindrical, tapering slightly toward pos- 
terior extremity. Males about 4 mm. long. Females about 10 mm. long; 
maximum diameter about 0.4 mm., although in some gravid females it is 
as great as 0.58 mm. Genital opening of female on ventral surface about 
0.1 mm. from the posterior extremity of the body. Posterior extremity 
of body about 0.19 mm. in diameter. Proboscis short, globular, usually 
slightly broader than long; length about 0.106 mm., diameter about 0.120 
mm. The eight hooks of terminal circle equal in size; not conspicuously 
