292 MOLLUSCOIDEA SUB-KINGDOM V 
a portion of their existence, or cemented ventrally. Valves ventral and dorsal. In 
composition, phosphatic or calcareous or both. Animal consisting of two pallial 
membranes intimately related to the shell. Within the mantle cavity at the sides of the 
mouth are inserted the two, more or less long, oral, usually spirally enrolled, cirrated 
brachia, which are variously modified, and are supported in the two terminal 
superfamilies by an internal calcareous skeleton, or brachidium, attached to the dorsal 
valve. Anus present or absent. Central nervous system consisting of an oesophageal 
ring with weakly developed brain and infra-oesophageal ganglionic swellings. Blood- 
vascular system probably present with the sinuses developed into vascular dilatations 
at the back of the stomach and elsewhere. Sexes separate. Exclusively marine. 
The class appeared in the Lower Cambrian, attained its maximum develop- 
ment in the Silurian and Devonian, and is represented by about 140 living 
species. Probably upwards of 6000 fossil and recent species have been 
developed ; these are distributed in 322 genera, grouped in thirty-one families, 
ten superfamilies, and four orders. 
Cuvier (1792 and 1802) was the first to distinguish the Brachiopods from 
the Acephala, and created for them a fourth family in his class of Molluses. 
To Duméril (1806) we owe the now generally accepted class name Brachio- 
poda, or arm-footed animals. Since the arms, or brachia, are not homologous 
with the foot of Molluscs, Gray (1821) changed the name to Spirobranchiopoda ; 
Blainville (1824) to Palliobranchiata.; Risso (1826) to Branchiopoda ; Broderip 
(1839) to Brachiopodidae ; Agassiz (1847) to branchionopoda ; Bronn (1862) to: 
Brachionocephala ; Paetel (1875) to Branchionobranchia ; and Haeckel to Spiro- 
branchia. None of these has displaced Duméril’s term, though founded on a 
false physiological interpretation of the brachia. 
External Characters: Form.—The shells of Brachiopods are very 
variable in form. Usually both valves are convex, but they may be nearly 
flat, with the interior cavity extremely shallow, or the dorsal valve may be 
concave and follow closely the curvature of the convex ventral valve. The 
ventral valve may be cone-shaped, with the dorsal operculiform, or the 
former may be so modified by cementation as to assume the appearance of a 
Cyathophyloid coral. The shell is commonly rostrate, with the ventral beak,. 
or apex, more or less incurved over that of the dorsal valve, or the valves 
may be very greatly extended transversely. In fact, the form of the shell of 
Brachiopods is so variable that, as a rule, no greater value than specific can 
be attached to this feature. 
Fizvation—The animal is generally attached to extraneous objects by a 
muscular pedicle which projects either from between the contracted posterior 
margin of the two valves (Fig. 499, 4), through an opening in or under the 
beak (Fig. 498, 6), or through the ventral valve (Fig. 512, 4). With 
increasing age, however, the pedicle opening frequently becomes closed, and 
the pedicle itself atrophied. The animal may then be anchored by spines 
(Chonetes, Productus) or be cemented by the whole or a part of the surface of 
the ventral valve (Crania, Davidsonia, Thecidea, Streptorhynchus). In some cases 
(Glottidia and Lingula) Brachiopods live throughout life partially buried in the 
sand or mud of the sea-bottom. 
Ornamentation.—The external form and ornamentation of the shell afford 
important characters for determining the species. The anterior margin of 
one valve is frequently indented by a median sinus, and the other usually 
exhibits a corresponding fold, or elevation. 
foe «. 
