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BRA CHIOPODA. 



A considerable part of the space between the valves of the shell is 

 filled up by two long " arms," which are coiled in a spiral, and often 

 supported by a calcareous skeleton. 

 These arise in development from the 

 specialisation of a horse-shoe shaped 

 "lophophore," such as is characteristic 

 of the Polyzoa. The mouth is placed 

 between the arms, and opens into the 

 ciliated food canal. This may end 

 blindly, or may be furnished with an 

 anus placed near the mouth ; in Crania 

 the anus is dorsal and posterior. The 

 muscular system is well-developed, the 

 shell is both opened and closed by 

 means of muscles. There is a nerve- 

 ring round the gullet, with a slight 

 brain and an inferior ganglion. Sen- 

 sory structures in many cases perforate 

 the valves. Above the gut lies the 

 heart, which is connected with blood 

 vessels. Two (or more rarely four) 

 nephridia open near the mouth, and 

 serve also as genital ducts. The pos- 

 terior region of the body often forms a stalk by which the shell is 

 moored, but in many this stalk is absent, and the animal is directly 

 attached to the substratum. The sexes are sometimes separate, but 

 perhaps some are hermaphrodite. There is a metamorphosis in the 

 development, and the larvre resemble those of Polyzoa. Of the details 

 little is yet known. 



Fig. 73.— Interior of Bra- 

 chiopod Shell, showing cal- 

 careous support for the 

 "arms." (After Davidson.) 



