292 MOLLUSCOIDEA SUB-KINGDOM v 



on of 11i< -if existence, or cemented re /if rail//. }'alret rent nil and dorsal. In 

 composition, phosphatic or calcareous or both. Animal consisting of two pallial 

 cs intimately rein to I f<> flic >/"'//. Jntlii.n. the inn idle cavity at the sides of the 

 inserted the two, 'more or less long, oral, usually spirally enrolled, cirrated 



In'ii f]i in, u-Jiirli are n/riously modified, and are supported in the two terminal 

 superfamilies by an internal calcareous skeleton, or brachidium, attached to the dorsad 

 valve. Anus^^resentjor absent. Central nervous system consisting of an oesophageal 

 ring with weakly developed brain and infra-oesophageal ganglionic swellings. ^Blooil- 

 vascular system probably present with the sinuses developed into vascular dilatation* 

 at the backoj'tlie 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 Molluscs. 

 To Dum6ril (1806) we owe the now generally accepted class name Bracliio- 

 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 JSranchionopoda ; Bronn (1862) to 

 Brachionocephala ; Paetel (1875) to Branchionobranchia ; and Haeckel to Spiro- 

 Iminchia. None of these has displaced Dum6ril'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 valvd 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 

 Cyathophylloid coral. The sjaelljs_cj3inni^ 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. 



Fixation. 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, A), through an opening in or under the 

 beak (Fig. 498, ), or through the ventral valve (Fig. 512, A). 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, TJiecidea, 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. 



