CHAPTER XIII 



The Lamp Shells 



{Phylum BracJiiopoda) 



A variety of lamp shells: (center) 

 lingula with its stalk in mud; ( top 

 right) hinged lamp shells clinging 

 a rock 



T 



iHE two parts of a brachiopod shell fit together 

 like saucers facing one another. But instead of these 

 being a right valve and a left, as in clams, one bra- 

 chiopod valve is dorsal, the other ventral. 



In most brachiopods, the ventral valve is somewhat 

 larger and more convex, and extends beyond the 

 dorsal valve around a definite opening. This gives the 

 shell a general form like that of the oil lamps of Greek 

 and Roman times, so often represented symbolically 

 as the "lamp of wisdom." The classic lamp was lit 

 through a hole corresponding in position to the one 

 through which a living brachiopod has a short stalk 

 serving to anchor the animal to some support. Usu- 

 ally the stalk holds the larger valve uppermost. 



The 260 or so existing species of lamp shells are 

 all marine. They represent today a slowly dwindling 

 line whose ancestors can be traced clearly in the fossil 

 record for 500 million years. During four-fifths of 

 that time, they have been in slow decline. About 

 3000 different extinct species of brachiopods are 

 known. 



Two genera of brachiopods hold the record for 

 surviving longer than any other group of animals. 

 Lingula, found first in the early strata of Ordovician 



age, goes back at least 350 million years, although 

 none of its existing species is particularly ancient. The 

 other genus. Crania, dates from late Ordovician to 

 the present, and is still millions of years older than 

 the next competitor. During Ordovician times, lamp 

 shells were more abundant than any other fossil- 

 producing type of animal. 



Inside its shell, each brachiopod shows a relation- 

 ship to bryozoans and phoronids in possessing a pair 

 of curled, tentacle-bearing arms, one on each side of 

 the mouth. Cilia on the tentacle surfaces create water 

 currents which enter at the sides of the shell, bringing 

 minute food particles and oxygen. Glands on the ten- 

 tacles secrete a mucus film in which food becomes 

 trapped. The loaded mucus is then swallowed. The 

 water leaves on the animal's midline, where the shell 

 valves gape most broadly when the muscles control- 

 ling them relax. 



Modern brachiopods include a few with shell valves 

 as much as 3 inches in greatest dimension. Some fos- 

 sil forms exceeded 1 foot across. Existing lamp shells 

 inhabit all seas at all latitudes, and find places at all 

 depths, from the intertidal sand flats to the great 

 abysses. Many of them are abundant locally. Some 



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