TAPEWORM PARASITIC IN THE STICKLEBACK. 133 
P. filaroides. In this ease though, the movements of two pairs 
of hooks alternate with those of a third pair, and the body 
changes from a spherical to a pear-shaped form. He also saw 
these movements in onchospheres of Hymenolepis nana. 
At this stage the onchosphere is swallowed by a Cyclops. It 
appears to pass through the stomach and first portion of the 
alimentary canal without any change or delay, and although it 
generally anchors itself to the wall about the junction of the 
thorax and abdomen, I have seen cases where it has attached 
itself to the wall of the alimentary canal just within the stomach 
on the one hand, and just above the anus on the other, but these 
are not common. After a certain time has elapsed, the extent of 
which is very variable but is usually a week, it breaks through 
the wall of the intestine into the dorsal sinus and is swept for- 
ward to lie usually near the posterior end of the carapace, but 
often in the head above the eye; it may also lie under the 
alimentary canal, but this is rare. Here it becomes very vacuo- 
lated at first, but later has a more solid appearance (PI. 1V. 
fig. 33). The hooks gradually disappear, and the body becomes 
partially covered with highly refractive granules, either isolated 
or disposed in clusters. These granules greatly resemble those 
found on young larve in the intestine of the stickleback, except 
that they are smaller. 
At the end of three weeks the larva is ready to be transferred 
to its second host (Pl. IV. fig. 37). It isan elongated grey body 
of variable size, and in situ seems to be an undeveloped part of 
the ovary. It is apparently studded with the refractive granules 
just mentioned, 004 mm. dia., which give it a characteristic 
appearance when removed from the Cyclops. In sections it may 
be seen that these really occupy spaces in the body parenchyma 
just under the cuticle and correspond to the calcareous bodies of 
older forms. There is no scolex and no neck, the body being 
entirely without divisions. Atthe anterior end are four suckers, 
‘03 mm. dia., which look like small blisters of the cuticle; they 
are probably functionless at this period. In sections they are 
practically level with the rest of the body. They are composed 
of the same parenchyma as the rest of the body (Pl. IV. fig. 35) 
only in a more compact form, and have two dark-staining types 
of cell. The first type is elongated, -005--007 x ‘001 mm. It is 
present only in larve in an advanced stage of development, and 
is probably an embryonal muscle-cell. The other type is the same 
in size and form as the cells of the parenchyma. In younger 
cases the suckers appear like the rest of the parenchyma, from 
which, in transverse sections, they are only separated by a thin 
line. There is no trace of the fifth sucker. The scolex is not 
invaginated at any time during the development; it appears as 
if it were gradually differentiated from the body parenchyma 
at an early stage. 
The whole body is covered by a cuticle, extending also over 
the suckers and excretory pore (PI. LV. fig. 36). Its outer layer 
