548 EVOLUTION OF ANIMALS Part V 



and an immensely long history all attested by their fossil remains. Un- 

 doubtedly, the adults were once free swimmers as their trochophore larvae are 

 now. However, through millions of years the adults proved the success of 

 their stalked food traps that contain a regulated collecting and filtering system 

 for gleaning particles of food from the water (Fig. 27.14). The fossils show 

 that their stalks extended from the posterior ends, as they do now, that the 

 shells opened upward, and that the long-lipped mouths expanded like the petals 

 of a flower. Their attached state and great abundance must have made them 

 food for roving predators, annelid worms, crustaceans, starfishes, and sea 

 snails. They constituted an important link in the food chain between the micro- 

 organisms they consumed and the carnivores that preyed upon them. 



Fossils of over 2500 species have been discovered and a large number of 

 these are known from Paleozoic rocks, the oldest rocks in which fossils of ani- 

 mals are found. The 225 living species are only a remnant of those that are 

 now extinct. Of the living brachiopods, Lingula is scarcely changed from its 

 ancient ancestors, an animal on which evolution paused (Fig. 27.15). 



Structure and Relationships. An adult brachiopod is enclosed within a pair 

 of shells resembling those of clams and oysters and like them, secreted by folds 

 of a fleshy mantle (Fig. 27.14). But the shells differ from those of mollusks in 

 that they cover the dorsal and ventral sides of the body, instead of the right 

 and left, and they swing open on a hinge at the rear end from which the body 

 stalk extends. In rock-dwelling brachiopods and most others, the shell is bent 

 upon the stalk. However, when they are burrowing, brachiopods hold their 

 bodies straight up, the original position with the tentacles and mouth facing 

 upward. 



Like bryozoans, a brachiopod has no real head, its place being taken by 



Digestive 

 Stalk gland 



Heart 



Adductor 

 yy\uscle 



Lophophore 



Storyiach 



Mouth 



MantU 



Fig. 27.14. Brachiopod, or lamp shell. A marine animal, about one inch long, 

 that superficially resembles a giant bryozoan crowded into a clam shell. Its im- 

 portance is in its antiquity, its residence on the ocean bottom over 400,000,000 

 years ago, and its pioneer development of kidneys (nephridia) and heart. (Cour- 

 tesy, Pauli: The World of Life. Boston, Houghton Mifflin Co., 1949.) 



