. Elements of Comparative Anatomy. NERVOUS SYSTEM OF BKACHIOPODA. 309 from tlie dorsal valve (Fig. 164, c). It serves as a support for tte arms. When first differentiated tlie shell is a soft chitinous layer, wliicla 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-co
. Elements of Comparative Anatomy. NERVOUS SYSTEM OF BKACHIOPODA. 309 from tlie dorsal valve (Fig. 164, c). It serves as a support for tte arms. When first differentiated tlie shell is a soft chitinous layer, wliicla 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 in 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 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 traversethe coelom, and serve to open and close, as well as to tui-n the shell (cf. Fig. 164). According to their function they traverse the ccclom 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. 161. Muscular system of Tereb rat ula. a h The two halves o£ the shell,
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