60 
COMPARATIVE ZOOLOGY. 
• mx 
Fig. 24.—Mouth of the 
Horse-fly (Tabanus lin- 
eola ): a, antennae; m, 
mandibles; mx, max¬ 
illae; mp, maxillary 
palpi; lb, labrum; l , 
labium, or tongue. 
As most of the Arachnids live by suo 
tion, the jaws are seldom used for masti¬ 
cation. In the Scorpion, the apparent 
representatives of the mandibles of an 
Insect are transformed into a pair of 
small forceps, and the palpi, so small in 
Insects, are developed into formidable 
claws: both of these organs are prehen¬ 
sile. In Spiders, the so-called mandi¬ 
bles, which move more or less vertically, 
end in a fang; and the club-like palpi, 
often resembling legs, have 
nothing to do with inges¬ 
tion or locomotion. Both 
Scorpions and Spiders have 
a soft upper lip, and a 
groove within the mouth, 
which serves as a canal 
while sucking their prey. 
The tongue is external, and 
situated between a pair of 
diminutive maxillae. 
In the Ascidians the first 
part of the alimentary canal 
is enormously enlarged and 
modified to serve as a gill- 
sac. At the bottom of this 
sac, and far removed from 
its external opening, lies 
the entrance to the diges¬ 
tive tract proper. Into it 
the particles of food enter- FlG 2 5 .—i 
ing with the water are con¬ 
veyed (Fig. 279). 
The mouth of Verte- 
Uncler Surface of Male Spider: a, 
c, poison-fang; b , teeth on interior mar¬ 
gin of mandible, e; /, labium; g, thorax ; 
h, limbs; i, abdomen; l, spinnerets; m, 
maxillary palpus; d , dilated terminal 
joint. 
