Larva of Chaoborus Crystallinus (de Geer). 349 
pair being attached by a short nerve to the region of the dorsal 
vessel occupied by the valves, and the remaining pair, also by a 
short nerve, to the vessel itself. These, in their turn, are con- 
nected by a peculiar X-shaped nerve, the free ends of which are in 
contact with the integument. 
We made out this structure for the first pair of valves, following 
the sac, and partly for the second and third pairs ; doubtless this 
structure repeats itself with each pair of valves. 
When making out these details, we seldom have used narcotics, 
but have preferred to allow the larva to remain in the live box for 
a period of thirty to forty minutes before commencing examination, 
during which time the creature becomes somewhat accustomed to 
its confinement. 
At first we made many unsuccessful attempts to get a sight of 
the corpuscles. When the larva is fresh and lively they shoot 
past the field of the microscope, and, as a rule, are out of focus, it 
being impossible to judge in what plane in the field they are likely 
to appear. With confinement in a live box comes a slowing down 
of action of the dorsal vessel; this in turn produces a sluggish 
flow of the blood or nutrient fluid, which brings about an accumu- 
lation of corpuscles in the neighbourhood of the entrance of the 
heart. A number of these can then be kept in focus, when it will 
be observed they slip away in twos and threes more often than 
singly, in response to the somewhat weakened pulsation. 
If you now turn to the valves already referred to you will 
have a better opportunity of observing their action. 
With a flick the Corethra can fling itself some distance, or, by 
a series of continuous and vigorous wriggles, descend from the top 
to the bottom of the aquarium. Obviously, such a delicate and 
important organ as the dorsal vessel needs, under such conditions, 
perfect attachments. 
We will now refer to a double row of small, and two large, 
chitinous hooks on the anal segment (fig. 9). These have been 
recorded by different writers, but the descriptions are not in agree- 
ment, nor altogether complete. “ Two chitinous hook-like bodies, 
and twelve small serrated plates” (ft)* “Two hook-like claws 
inserted on each side of the rectal opening ” (5). “ Limbs would 
be superfluous, an,d nothing can be seen of them except a promin- 
ence provided with a double crown of minute teeth which can be 
distinguished by the microscope at the extreme end of the tail ” (f). 
u Corethra larva transparent, last segment usually with hooks ” (6). 
“ There is a circle of setae on the anal segment ” (7). From most 
of the foregoing references the normal position of these hooks 
would appear to be exterior on the anal segment and fixed. 
In all live specimens, in a normal state, these hooks are neatly 
The figures in brackets refer to the bibliography at the end of the paper. 
