NERVOUS SYSTEM OF LKACHlOrODA. 



309 



from the dorsal valve (Fig. 164, c). It serves as a support for the 

 arms. 



When first differentiated the shell is a soft chitinous layer, which 

 later on becomes calcified. The valves are traversed by pore-canals, 

 which are filled up by villous processes of the mantle. Between these 

 the firm substance of the shell is seen to be composed of prismatic 

 bodies, which can be made out even when the shell is first laid down ; 

 they are set obliquely to the edge of the shell. 



Owing to the great increase in surface of the spirally-coiled 

 arms of the Brachiopoda, thanks to their investment of tentacles, 

 they form organs well adapted for the respiratory function. In the 

 first place the tentacular filaments are suitably arranged for respira- 

 tion. They are iu communication with the blood-sinuses which pass 

 along the arms. In their functional relation they may therefore be 

 regarded as gills. The two arms are connected with one another at 

 their bases, which are directed towards the middle line. A fold 

 above the mouth extends on either side on to the arms, and aids in 

 marking off a groove, which extends from the arms to the mouth. 

 The tentacles or cirri, which are arranged in two rows, and closely 

 approximated, rise up on the other edge of this 

 groove ; they extend to the end of the arms. 



Muscular System. 



§ 241. 



Besides the muscles of the dermo-muscular 

 tube, such as those of the mouth and of the 

 arms, we find in the Brachiopoda a number of 

 independent muscles, which traverse the ccelom, 

 and servo to open and close, as well as to turn 

 the shell (cf. Fig. 164). According to their 

 function they traverse the ccelom in different 

 directions, and arise from, as well as insert 

 themselves into, the valves of the shell, so that they may be regarded 

 as differentiations of the dermo-muscular tube, which were formed 

 when these valves were formed. 



Fig. 164 Muscular 

 system of Terebratula. 

 a h The two halves of 

 the shell, c Support for 

 the arms, d Stalk, cf 

 g h Muscles for opening 

 and closing the shell 

 (after Owen). 



Nervous System and Sensory Organs". 



§ 242. 



The nervous system is very peculiar ; it alone would justify iis 

 in giving the Brachiopoda an independent position. It is made up 



