400 
Digenetic Trematodes in Japan 
specimens are met with. Similar young specimens are often found in the 
air-bladder. It seems probable, therefore, that the coelom is not a favourable 
situation for the maturation of this species. 
A cercaria form, which seems to belong to this species, I shall reserve for 
future description. 
(18) Distomum sp. from Thynnus thynnus. 
(A species near Distomum clavatum.) 
This species infests the stomach of Thynnus thynnus. One somewhat 
immature specimen was obtained from a fish in the Tokvo market. 
It is roughly cylindrical in shape, the widest part being situated near the 
posterior end of the body. Anteriorly it has an almost uniform breadth and 
terminates in a rounded extremity, while posteriorly it tapers rapidly. The 
size of a moderately contracted specimen is 25 mm. long and 5 mm. wide in 
the widest part. The oral sucker is situated at the anterior end of the body, 
the aperture pointing forwards. It is cup-shaped, measuring 1*1 mm. both 
in length and dorso-ventral diameter. The ventral sucker lies about 4 mm. 
posterior to the anterior end. It is protruded remarkably and even peduncu¬ 
lated. The shape is a deep cup-form, and the main axis of the sucker runs 
obliquely. The length of the sucker is 2 mm. and the aperture of the lumen 
is 1 mm. in longitudinal diameter. In fixed specimens the anterior part of 
the body is bent dorsally at the level of the ventral sucker. 
The cuticle is very thick and unarmed. The pharynx lies directly posterior 
to the oral sucker. It is spherical, the diameter being 0*7 mm. The oesophagus 
curves dorsally, its length being about 0*6 mm., and from it two pairs of the 
intestinal caeca arise, the anterior and the posterior. The anterior ramus is 
short, ending near the dorsal part of the pharynx; the posterior ramus reaches 
almost to the posterior end of the body and has numerous mesial and lateral 
branchings. The lateral branches are further subdivided. 
The excretory vesicle opens at the posterior end of the body. Its unpaired 
part runs forward for about 5 mm. and then divides into two (?) arms. The 
unpaired part lies between the two intestinal caeca, its wall following an 
uneven course, alternating with the branches of the intestine. The lateral 
arms run posteriorly and, when nearing the posterior end of the body, they 
turn anteriorly and reach the dorsal part of the oral sucker. It seems probable 
that they run back again posteriorly. In their course numerous slight undula¬ 
tions are found. 
The two testes lie in the median line immediately posterior to the ventral 
sucker, the anterior one overlapping the posterior. Their shape is spheroidal, 
the diameter being about (K5-0-6 mm. Anterior to the ventral sucker is the 
vesicula seminalis. It is a broad tube and runs along the median plane, 
winding laterally several times. The pars prostatica is a long winding tube 
w r hich leads to the ductus ejaculatorius. There is no cirrus-pouch. The ductus 
ejaculatorius opens on the genital papilla wdiieh lies in the genital atrium. 
