154 . INVERTEBRATE ANIMALS, 
The pulmonary sacs are two or four in number, and open on 
the surface of the abdomen by as many apertures. The head 
bears from six to eight simple eyes; the mandibles are hooked, 
and carry the duct of a poison-gland; and the maxillary palpi 
are not developed into nipping-claws. The spiders are all pre- 
daceous and rapacious animals, and many of them possess the 
power of constructing webs, either for the capture of their prey, 
or simply for lining their habitations. For the production of 
the web, spiders are furnished with special glands situated at 
the extremity of the abdomen. The secretion of these glands is 
a viscid fluid, which hardens rapidly on exposure to air, and 
which is cast into its proper thread-like shape by passing 
through what are called the ‘‘spinnerets” (fig. 71, 4.5). These 
are little conical or cylindrical organs placed at the end of the 
abdomen, and perforated by a number of extremely minute 
tubes, through which the secretion of the gland has to pass 
before reaching the air. Many spiders, however, do not con- 
struct any web, unless it be for their own habitations, but = 
hunt their prey for themselves. 
_ The spiders are oviparous, and their young pass through no 
metamorphosis, but they cast their skin or “ moult” repeatedly 
before they attain the size of the adult. 
