166 NEW-YORK FAUNA MOLLUSCA. 



ORDER IV. ACEPHALA. 



Body fixed or free. No distinct head, but a mouth without teeth, concealed in the bottom or 

 between the folds of the mantle, often furnished on each side with a pair of appendices- 

 Eyes none. The gills usually consist of four large lamina,, or leaflets, with a vascular 

 network. Sexes united in the same individual. Aquatic. The shell external, and mostly 

 composed of two valves, or wanting, but in that case furnished with a thick mantle. 



Obs. The animals of this order are divided by Cuvier into two sections : the first, which 

 is most numerous, contains all the bivalve and some of the multivalve shells ; the other, 

 Acephala nuda, comprises those in which the shell is replaced by a cartilaginous membrane. 

 We shall consider his class Brachiopoda as a section of the Acephala, with which they have 

 many characters in common ; although the position of the animal in its shell, with its back 

 against the hinge, differs from other bivalves. 



SECTION 1. BRACHIOPODA. 



Animal enveloped in a bilobed mantle, which is always open. Mouth anterior, and furnished 

 with a pair of fleshy arms with curled filaments at their edges, and capable of being ex- 

 tended externally. The gills applied to the internal surface of the lobes of the mantle. 

 Vent anterior. Organs of generation unknown. Shell bivalve, united behind either 

 with or without a hinge, opening in front. 



FAMILY TEREBRATVLIDM. 



Animal more or less globular or flattened, with the montle open in front and towards the side. 

 Shell inequivalve, equilateral, with a hinge, and adhering to other bodies either directly 

 or by means of a tendinous cord. 



EXTRA LIMITAL. 



Genus Terebratula, Bruguieres. Animal with the gills arranged in a pectinated form on the inner 

 surface of the mantle ; the long arms rolled into a spiral form when at rest. Shell variable in 

 its form, often ribbed : one valve prolonged into a recurved beak, and perforated at its tip, for 

 the passage of a ligament, by which it attaches itself to foreign bodies ; two bony processes 

 on the interior of the smaller valve. 



