BRACHIOPODA. 175 



The nervous system consists of a ring around the oesophagus 

 with two swellings or ganglia in it ; nerves are given off to the 

 rest of the body. 



It is apparent that the pedicle valve is so called because the 

 pedicle passes out through it and the brachial valve is so named 

 because the arms or brachia are fastened to it. 



Brachiopods are, in the larval stage, free-floating or mero- 

 planktonic, and hence it is during this period of their lives that 

 their distribution takes place. 



Brachiopods appear in the Lower Cambric ; they reach their 

 maximum in the Siluric and Devonic and continue to the present. 

 There are about 6,000 fossil species known and 140 recent. 



These shells often appear very similar to Pelecypods but may be 

 distinguished from them by the following characters, which as a 

 rule hold true : 



Brachiopods. Pelecypods. 



Equilateral. Inequilateral. 



Inequivalved. Equivalved (generally). 



Pedicle opening present (except in No pedicle opening present. 



Atremata ) . 



Teeth in one valve, sockets in the Teeth and sockets in each valve 



opposite valve (except Inarlic- (typically). 



ulata). * 



No ligament present ; valves opened Valves opened by ligament or resilium 



by muscles. at hinge line. 



Brachiopods are divided according to the presence or absence 

 of teeth into two subclasses : 



L Inarticulata — Tzzth. absent. Valves held in apposition by 

 muscles. This is subdivided into two orders : 



1. Atremata — Pedicle emerging from between the valves and 

 usually with no pedicle opening present. 



2. Neotremata — Pedicle opening confined to pedicle valve and 

 modified by false deltidium or by listrium. 



II. Articulata — Valves articulated by teeth and sockets. This 

 is subdivided into two orders : 



1. Protremata — Pedicle opening restricted to pedicle valve and 

 modified by true deltidium. Brachia have no calcareous supports 

 except very short ones in Pentamerus. 



2. Telotremata — Pedicle opening shared by both valves in 

 early life and usually confined to one in later stages. Brachia 

 supported by calcareous supports. Deltidial plates usually present. 



