Xx : LIFE-HISTORY 493 
organs that assist in introducing the spermatozoa into the 
female. The two tubes then unite, and having received a 
dorsally-placed accessory gland, open to the exterior by a median 
aperture placed ventrally a little way behind the mouth. 
Life - history— The egg undergoes a large portion of its 
development within the body of the mother. In Linguatula 
taenioides, Which lives in the nasal cavities of the dog, the eggs 
pass away with the nasal excretions. If these, scattered about 
in the grass, etc., be eaten by a rabbit, the egg-shell is dissolved 
in the stomach of the second host and a small larva is set free. 
In Porocephalus proboscideus and others, which inhabit the lungs 
of snakes, the eggs pass along the alimentary canal and leave the 
body with the faeces. They also must be eaten by a second host 
if development is to proceed. 
The larva which emerges when the egg-shell is dissolved has 
a rounded body provided with two pairs of hooked appendages, 
and a tail which is more 
or less prominent in 
different species (Figs. 
259, 260). Each ap- 
pendage bears a_ claw, 
and is strengthened by 
a supporting rod or i 
skeleton. Anteriorly the |\. °°: i EES 
head bears a boring ap- \[ 
paratus of several chitin- 
ous stylets. The various 
internal organs are in 
this stage already formed, 
though in a somewhat 
°o Pr Ly, 
oi d 
: -, Fic. 259.—A late larval stage of Porocephalus pro- 
rudimentary state, and it boscideus, seen from the side. Highly maguified. 
is doubtful if the anus (From Stiles.) 1, primordium of first pair of 
chitinous processes; 2, primordium of second 
has yet appeared. pair of chitinous processes ; 3, mouth : 4, ventral 
Fs EE erat eet naga 
apparatus, and aided by 
its hooked limbs, the larva now works its way through the 
stomach-walls of its second host, and comes to rest in the liver 
or in some other viscus. Its presence in the tissues of its second 
host causes the formation of a cyst, and within this the larva 
rests and develops. In man, at least, the cysts often undergo a 
