70 ANIMAL FORMS 
72. Lamp-shells or Brachiopods—Occasionally one may 
find cast on the beach or entangled in the fishermen’s 
lines or nets a curious bivalve animal similar to the form 
shown in Fig. 438. ,These are the Brachiopods, or lamp- 
shells. The remains of closely related forms are often 
abundant as fossils in the rocks (Fig. 43). Over a thousand 
species have been preserved in this way, and we know that 
in ages past they flourished in almost incredible numbers 
and were scattered widely over the earth. Unable to adapt 
themselves to changing conditions or unable to cope with 
their enemies, they have gradually become extinct, until 
to-day scarcely more than one hundred species are known. 
These are often of local distribution, and many are com- 
paratively rare. 
For a long period the Brachiopods, owing to their pecul- 
iar shells, were classed together with the clams and other 
bivalve mollusks. The presence of a mantle also strength- 
ened the belief; but closer examination during more recent 
years has shown that the shells are dorsal and ventral, and 
not arranged against the sides of the animal as in the 
clams. Another peculiar structure consists of two great 
spirally coiled “arms,” which are comparable in a general 
way to greatly expanded lips. The cilia on these create, in 
the water currents which sweep into the mouth, the small 
animals and plants that serve as food. The internal organ- 
ization resembles in a broad way that of the animals con- 
sidered in the previous section, and it now appears that 
both trace their ancestry back to the early segmented 
worms. 
73. Band or nemertean worms.—In a few cases band or 
nemertean worms have been discovered in damp soil or in 
fresh-water streams. These are commonly small and incon- 
spicuous, and are pigmies when compared with their marine 
relatives, which sometimes reach a length of from fifty to 
eighty feet. Many of the marine species (Fig. 44) are often 
found on the seashore under rocks that have been exposed 
