PYCNOGONIDA. 339 
The type of this class is the spider, which is character- 
ized by the pos- 
session of two 
or three pairs 
of spinnerets, 
which are 
jointed ap- 
pendages ho- 
mologous with 
the legs. Be- 
sides trachex, 
spiders have a 
Fig. 366.—Anatomy of a spider, diagrammatic longitudinal so-called lung 
section through the body. au, simple eyes and nerves leading (Fi 306 L) 
to them from the brain (supra-cesophageal ganglion. 0G); & o 
Gis, mandibles; ¢a@_, palpus of maxilla /,5 Jp, ae of legs, COM pose d of 
, succeeding pairs; K, head; B7, thorax ; hind-body 
or abdomen; Ru, heart or dorsal vessel ; Z, lung in front of several leaves, 
the opening of the oviduct @; the spinning-glands (sp) con- . ° 
nect with the spinnerets, sp W. The digestive tract is »haded, into which the 
and in the abdomen enveloped in the liver.—After Graber. 
blood flows, 
and is thus aérated. In Lycosa the blood flows through the 
heart from the head backward. There is a great range of 
structure, from the lowest mites to the spiders, certain mites 
having no heart, no trachez, very : 
rudimentary mouth-parts, and no 
brain, there being but a single 
ganglion in the abdomen. 
Order 1.—The Pycnogonida 
are marine forms, without air- 
tubes, with four pairs of long 
legs, into which coecal prolonga- 
tions of the stomach pass, as seen 
in Fig. 307. 
Order 2. Tardigrada*—The 
bear animalcules (Fig. 308) are 
related to the mites. In these Fig. 307.—Ammothod pycnogo- 
. im . noides. a, stomach with ceca (6, 
singular beings the ovary and 4, 6,d) extending into the legs.— 
testis exist in the same individual. *"°™ Gesenbaur. 
Macrobiotus Americanus Pack. is common in sphagnum 
swamps. Like the Rotatoria, these low forms are capable 
of revivifying after being apparently dead and dried up. 
* The Pycnogonida, Tardigrada, and Linguatulina are probably 
independent classes of Arthropods. 
