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. 43. 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 



