THE BRACHIOPODA. 397 



to the arms of Rhabdoplexira^ though these, like the arms 

 of the Brachiopoda, are probably more strictly comparable 

 to the labial palpi of the Lamellibranchs. 



Polyzoa occur in the fossil state from the Silurian epoch 

 to the present day, and the oldest forms are referable to the 

 groups which now exist. 



The Buachiopoda. — The Brachiopoda are all marine 

 animals provided with a bivalve shell, and are usually fixed 

 by a peduncle which passes between the two valves in the 

 centre of the hinge line, or the region which answers to it, in 

 those Brachiopods which have no proper hinge. They never 

 multiply by gemmation, nor give rise to compound organisms. 

 The shell is always inequivalve and equilateral ; that is to say, 

 each valve is symmetrical within itself and more or less un- 

 like the other valve. The shell is a cuticular structure se- 

 creted by the ectoderm, and consists of a membranous basis, 

 hardened by the deposit of calcareous salts, sometimes con- 

 taining a large proportion of phospliate of lime {Lingula). 



In many Brachiopods, variously-formed calcareous spic- 

 ula, or minute plates, are found in the walls of the peri- 

 visceral cavity, and of the greater sinuses ; and also in the 

 arms and cirri, and sometimes these unite together so as to 

 form an almost continuous skeleton.^ 



The body, or rather that part of it which contains the chief 

 viscera, is often small relatively to the valves of the shell, 

 and the integument is produced into t>vo broad lobes, which 

 line so much of the interior of the valves as the visceral mass 

 does not occupy. The free edges of these lobes are thickened, 

 and are beset with numerous fine chitinous setae like those 

 found in Annelids, and like them lodged in sacs. Between 

 the two lobes of the mantle, or palliuyn^ is the pallial cham- 

 ber, bounded behind the anterior wall of the visceral mass. 

 In the middle line, this wall presents the oral aperture, which 

 is seated in the midst of a wider or narrower area, the mar- 

 gins of which are provided with numerous ciliated tentacula. 



In Argiope, the oral area occupies a laro;epart of that lobe 

 of the mantle wliich is ordinarily termed dorsal, and its mar- 

 gins are simply indented by three deep sinuations. In Theci- 

 diunij the sinuations are deeper, and the folds of the oral area 

 thus produced narrower. But in most Brachiopods the oral 



1 These have been described by Woodward, Lacaze-Duthiers, aud especially 

 by Eudes Desloiifrchamps, " Recherches sur 1' organisation du Manteau chez les 

 Brachiopodes articules," 18G4 



