338 Prof. R, Leuckart on the Developmental 
gin of the buccal orifice; but the latter projects far more dis- 
tinctly, and is also larger. The cesophagus is still free from the 
subsequent division into the muscular and glandular portions. 
The beaker-shaped armature of the mouth is also wanting; in 
its place there is only a short and narrow chitinous tube leading 
into the interior. 
The further development of the embryos takes place in the 
little Cyclopes which inhabit our waters in such quantities (more 
rarely in the larvee of the dragonflies). Within a few hours of 
the transference of these animals ito the aquaria stocked with 
the young worms, a few immigrants may be found in their in- 
terior. The number increases in course of time up to several 
dozens ; indeed I once found thirty-four young Cucullani toge- 
ther in a Cyclops of only moderate size. The worms, owing to 
the toughness of their cuticle, can resist the action of the water 
for a long time (for single embryos may be seen moving about 
briskly in the mud a fortnight after their mtroduction) ; hence 
their immigration may take place with tolerable certainty even 
in freedom. 
From the presence of the boring-tooth on the upper hip, it 
might be supposed that the immigration takes place directly 
through the external integuments, as in the case of the Cercarie ; 
but, as far as I have been able to observe, this is not the case. 
The embryos are rather taken up through the mouth of the 
Cyclops. They are first seen in the intestine, and only sub- 
sequently in the body-cavity, where they twist about briskly for 
a time between the muscular cords and the intestines. 
Gradually, however, this movement becomes slower. The 
worm imcreases In length (to 0-6 millim.) and diameter, and 
after a certain period. (in summer within three days; in winter 
later) undergoes a change of skin, after which it exchanges its 
previous slender form at once for a plump one. On closer ex- 
amination, moreover, many differences from its previous struc- 
ture are detected. Thus the boring-tooth disappears with the 
old skin, and the tail is reduced nearly to one-half of its previous 
length. The cesophagus has become divided, by a stronger de- 
velopment of its posterior half, into two sections differing from 
each other even histologically, and which constitute almost one- 
third of the whole intestinal canal (elsewhere of a deep brown 
colour). The outer walls of the body have also increased con- 
siderably in thickness, although the cuticle is perhaps thinner 
than before. 
After the worms have remained in this state for some time, a 
new change takes place in them. This is the preparation for 
the formation of the buccal cup. The rounded anterior extremity 
of the cesophagus which issues outward through a short, funnel- 
