BY J AS. P. HILL. 
5 
sacs are also concealed from view. The gonads in sexually 
mature animals are found in the wings almost immediately behind 
the posterior rim of the collar, and they extend in the ridges far 
into the hepatic region. In sexually mature individuals the gonads 
may distend the genital wings to such a degree that they stand 
out almost horizontally to the body (fig. 23, giv.) and leave the 
gill area exposed, but the usual condition is the one first described 
where the gill area is completely concealed. 
The gill area may reach a length of 15 mm., and is thus 
relatively very short. It is broadest just behind the collar, and 
narrows slightly posteriorly; laterally it is bounded by two longi- 
tudinal grooves which become gradually shallower posteriorly and 
terminate at the end of the gill area. Into these the gill pores 
open. 
(2) Hepatic Region: varies in length according to the 
size and age of the animal. In one of my largest preserved 
specimens ib measured over 20 mm. in length with about 50 liver 
sacs on each side. The number of sacs averages between 40 and 
50 on each side, but there may be as many as 60. In fully 
extended animals the liver sacs present a distinctly paired 
arrangement in two longitudinal rows. They begin as small 
elevations in contact with each other, and covered by the genital 
wings; posteriorly they gradually increase in size, and attain 
their maximum development just beyond the point where the 
genital wings are reduced to ridges. Behind this the sacs 
gradually decrease in size to the posterior end of the region, 
finally being represented by two rows of small tubercles which 
become gradually reduced until they disappear altogether. Each 
sac arises by a narrow base which widens out into an antero- 
posteriorly compressed end, which in the region of the larger sacs 
overhangs the reduced genital wings. The form of the sac in this 
region is shown in fig. 24, hep. 
Variation i n Hepatic R e g i o n. The sacs are 
usually in close contact with each other by their anterior and 
posterior faces, but occasionally some of the creca present the 
