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. 
Oss. 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. 
Anima 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 ea- 
tended externally. The gills applied to the internal surface of the lobes of the mantle. 
Vent anterior. Organs of generation unknown. Suet bwalve, united behind either 
with or without a linge, opening in front. 
FAMILY TEREBRATULID:. 
Anima 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. 
