1004 ELLIOT ROWLAND DOWNING 
They are taken from typical nephridia and are drawn to the 
same scale. In each case the individual selected was an aver- 
age sized worm and was taken at a time of the year, too, when 
the particular species was approaching the maximum of its breed- 
ing activity, so the gonad should have its maximum size. The 
extent of the gonad evidently varies greatly. It is confined to a 
relatively small area on the gonadial vessel in A. grubii and clap- 
aredii. Itismuch more extensive in A. cristata and about equally 
soin A. marina. The gonad achieves its greatest size in A. ecau- 
data. The relatively immense testes of this species are due to the 
fact that the sperm are retained within them until nearly mature 
while in the other species the spermatogonia are early discharged 
into the body fluid, there to undergo the major part of their devel- 
opment. The large bladders of the nephridia of A. grubil seem 
similarly due to the fact that they are used as storage rooms for 
the sexual products after their formation, while in A. cristata, 
marina and claparedii these are held in the general body cavity. 
LIMITS OF THE GONADS 
The gonad usually surrounds the blood vessel, It appeazs as 
a light yellow incrustation, varying in thickness with the season. 
As it decreases in size it occupies a more and more restricted area 
on the posterior portion of the gonadial vessel. Not infrequently 
the other blood vessels adjacent to the nephridia, the dorsal longi- 
tudinal, the nephridial longitudinal and the nephridial branch of the 
afferent, are covered with a similarly appearing incrustation, but 
on microscopic examination, the material is found to be chloro- 
gonous, never gonadial. Furthermore, the gonad is confined to 
the blood vessels of the second to the fifth nephridia, inclusive, in 
A. cristata. Over a hundred males have been carefully examined: 
in 6.2 per cent, one or both of the first nephridia had the gonadial 
vessel slightly to plainly coated with the yellowish incrustation; 
11.8 per cent of them had the gonadial vessel of the sixth nephrid- 
ium coated. Such nephridia have in all cases been removed and 
sectioned and with one possible exception, the sections show the 
incrustation to be chlorogonous. In a single instance the gonad- 
ial vessel of a sixth nephridium had upon it a few cells looking 
