^jS RECENT BRACHIOPODA 



CHAP. 



The Nervous System 



The nervous system of Biacliiopods is not very clearly 

 understood, and there are considerable discrepancies in the 

 accounts of the various investigators, even when they are 

 dealing with the same species. So much, however, seems certain, 

 that there is a nervous ring surrounding the oesophagus, that 

 this ring is enlarged dorsally, or, in other words, near the base 

 of the lip, into a small and inconspicuous dorsal ganglion, and 

 again ventrally or just behind the base of the tentacles into a 

 ventral or sub-oesophageal ganglion. The latter is, contrary to 

 what is usual in Invertebrates, of much larger size than the 

 supra-oesophageal ganglion, but like the last named, it has re- 

 tained its primitive connexion with the ectoderm or outermost 

 layer of the skin. Both ganglia give off a nerve on each side 

 which runs to the arms and along the base of the tentacles and 

 lips. The sub-oesophageal ganglion also gives off nerves which 

 supply the dorsal and ventral folds of the mantle, the muscles, 

 and other parts. 



The modified epithelium in connexion with the ganglia may 

 possibly have some olfactory or tactile function, but beyond this 

 the Brachiopoda would appear to be devoid of eyes, ears, or any 

 other kind of sense organs, — a condition of things doubtless 

 correlated with their sessile habits, and with the presence of a 

 bivalved shell which leaves no part of their body exposed. 



The Reproductive System 



The majority of Brachiopods are bisexual, and many autho- 

 rities regard the separation of sex as characteristic of the group ; 

 on the other hand, Lingula pyramidata is stated to be herma- 

 phrodite, and it is not impossible that other species are in the 

 same condition. 



The generative organs are of the typical sort, that is, they 

 are formed from modified mesoblastic cells lining the body 

 cavity. These cells are heaped up, usually in four places, and 

 form the four ovaries or testes as the case may be (Fig. 314). 

 The generative glands usually lie partly in the general body 

 cavity and partly in the dorsal and ventral mantle folds, two on 

 each side of the body. Along the axis of the heaped-up cells 



