BRACHIOPODA. 



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tractile cirrhi which the latter have on the border of 

 the mantle, the Brachiopoda have at the edge very brittle, 

 glassy, stiff filaments, which are hollow, and take deep 

 root in the substance of the mantle ; they are very long 

 in the Discina and Lingula, very small in Terebrutula. 

 A very complicated system of muscles serves to close the 

 valves; in most cases four pair of closing or adductor 

 muscles are distinguished, of which one pair only is in- 

 serted at both ends into the valves ; the others are fixed 

 by one end to the shell, and by the other unite them- 

 selves to the tendon, and serve to attach the animal 

 to any foreign body. The impression of these muscles is 

 therefore usually very distinct in both valves. The move- 

 ment of the arm-like tentacles is produced by a peculiar 

 apparatus; the fringes of these tentacles are set upon a 

 cartilaginous, tubular, hollowed projection, diminishing to- 

 wards its point. In the hollow of this projection, which 

 is closed at both ends, a fluid is contained which, by the 

 contraction of the circular muscles at their bases, is driven 

 up to the point, by which means it stands erect, and seems 

 to force the valves of the shell asunder. These tentacular 

 arms in many Brachiopoda are situated upon a very re- 

 markable, inner, calcareous or bony formation, which always 

 originates in the imperforated valve, and is much diver- 

 sified. In some two thin stalks run inwards from the 

 hinge-teeth, which support a more or less complicated 

 ring; sometimes there is a central perpendicular plate, 

 which forms a sort of imperfect wall of separation; at 

 others this sends off wonderfully developed arms, which 

 frequently unite themselves with the arms issuing from 

 the hinge teeth, and form a very complicated frame. 

 No organ analogous to the foot of other Molluscs exists. 

 The stalk by which many of the Brachiopoda are held 

 fast, answers to that part in Anomia, and consists of a 



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