BRACIIIOrODA. 



485 



BRACHIOPODA. 



The Brachiopods, like the Ascidian?, are deprived of the power of 

 h)coraotion, and are attached by their shell or by a peduncle to 

 foreign bodies. Their muscular tunic or mantle is, as it were, slit 

 open, and consists of two broad membranous expansions, called " pal- 

 lial" lobes, which are covered by, and closely adhere to, two cal- 

 careous plates, adapted to enclose and defend all the soft parts of the 

 animal. The branchial sac may be supposed to be equally cleft and 

 adherent to the muscular one, so as to form the internal vascular 

 layer of the pallial lobe. 



The Brachiopods have left their testaceous remains in the most 

 ancient deposits known to contain evidence of animal life : they flou- 

 rished during the palteozoic and secondary periods, and are most 

 abundant, and exemplified by the most varied forms, in the fossil 

 state. They are now, of all Mollusks, the most widely diffused over 

 tiie earth's surface, and they can exist at greater depths than most 

 other bivalves : they are thus amongst the oldest of existing forms of 

 animal life, and their range in time is as extensive as in space. 



Of the few existing genera of this singular and beautiful order of 

 shell-fish, the Lingula is characterised by its long peduncle, and by 

 the equality of the valves of its shell, neither of which are perforated : 

 the Orbicula {Discinrr, Lam.) is sessile, and adheres by one end of a 

 short transverse muscle, which perforates the ventral valve of the 

 shell, which is the flatter valve. The Terehratula {figs. 186 and 187) 

 is attached by a short peduncle (b), which projects through a hole in 

 a beak -shaped prolongation of the ventral valve (v), which is the more 

 convex one. To the dorsal valve (d) is attached the internal cal- 

 careous process, usually in the form of an elastic loop, and called the 



apophysary system (a, b, c, d^Jxg. 

 187). The subdivision of the 

 genus Terebratula*, the most cha- 

 racteristic of the Brachiopods, and 

 the type of the most extensive 

 family of the order, is based chiefly 

 on modifications of the internal 

 processes. The foramen in the 

 ventral valve is usually more or 

 less formed by a small detached part 

 called the " deltidium." The valves are articulated by two curved 



Terebratula tlarescens. 



* Dim. of lerebratus, perforated. 

 113 



