1590 INVERTEBRATE ANIMALS, 
the antenne of the Crustacea. In the lower Arachnida, suchas 
the ticks, the organs of the mouth are modified to enable them 
to imbibe fluids. 
Fig 71.—Morphology of Arachnida. 1. Organs of the mouth in the scorpion on 
one side; 72 Mandibles (antennz) converted into pincers; #4 Appendages of 
the maxillz (maxillary palpi) greatly developed, and constituting nipping-claws. 
2. Telson, or terminal joint of the abdomen, in the scorpion. 3. One of the 
abdominal segments in the scorpion, showing the ‘‘spiracles” or apertures of 
the pulmonary sacs. 4. Tegenaria domestica, the common spider (male), 
viewed from below; s Spinnerets; #2 Mandibles with their perforated hooks, 
—below the mandibles are the maxilla, and between these the lower lip; # 
Maxillary palpi. 
The mouth in the Avachuida opens into a pharynx, which 
is of extraordinary small diameter in the true spiders, which 
live simply on the juices of their prey. The intestinal canal is 
usually short and straight, and is continued without convolu- 
tions to the aperture of the anus. Salivary glands are also 
present, as well as ramified tubes which are believed to act as 
kidneys. 
The circulation is maintained by means of a dorsal heart, 
which is situated above the alimentary canal. All the Arach- 
nida, except, perhaps, some of the lowest, breathe air directly, 
and the function of respiration is performed by the general sur- 
face of the body (as in the lowest members of the class), or by 
branched air-tubes termed “ trachez,” or by distinct pulmonary 
chambers or sacs, or, lastly, by a combination of trachez with 
