64 THE OPEN BOOK OF NATURE 
state. Some of them live in tubes of lime, and such 
worm-tubes are sometimes discovered as fossils. 
You often see worm-tubes on stones and shells on 
the seashore, but they, of course, are not fossils. 
Burrows and trails of worms occur in some very old 
rocks. 
The next subdivision that we have to consider 
is that of the AnrHROPOoDA (Greek, arthron=a joint, 
podos=a foot). This great subdivision includes 
Crabs, Lobsters, Spiders, Insects, Scorpions, Centi- 
Spheronites Litchi Pseudocrinites quad- Hchinospherites auran- 
(Ordovician). rifasciatus (Silurian). téum (Ordovician). 
Fig. 14.—Fossin CystipEANs. 
pedes, etc. All these animals’ bodies are divided 
into a number of segments, each segment bearing a 
pair of limbs, and the limbs are usually jointed. 
The animals also have elaborate digestive, circu- 
latory, and nervous systems. Their fossil remains 
are widely distributed. 
This sub-kingdom is arranged into four great 
classes : 
1. Crustacea (Latin, crusta=a rind).—Head and 
thorax united and covered with a carapace—that is, 
