THE DEVELOPMENT OF THE BRACI1IOPODA. 401 



The nervous system of the articulated Brachiopods, in 

 which it has been best made out, consists of a relatively thick 

 ganglionic band on the ventral side of the mouth, the ends of 

 which are united by a commissural cord, which surrounds the 

 gullet, and bears two small ganglionic enlargements. The lat- 

 ter probably answer to the cerebral, the former to the pedal, 

 ganglia of the Lamellibranchiata. Immediately behind the 

 pedal mass, from which two large nerves to the dorsal or ante- 

 rior lobe of the mantle are given off, are two elongated ganglia, 

 connected by a commissure of their own, which possibly cor- 

 respond with the parieto-splanchnic ganglia of the higher Mol- 

 lusks. The nerves to the ventral lobe of the mantle and those 

 to the peduncle arise from these ganglia. 



In the inarticulated Brachiopods, our knowledge of the ner- 

 vous system is very imperfect. In Lingula, Professor Owen 

 has described two lateral nerve-cords, and the observation 

 has been confirmed by Gratioiet and Morse. The latter anato- 

 mist finds similar cords in Discina, and Gratioiet has de- 

 scribed an oesophageal ring in Lingula. 1 



The reproductive organs are lodged in the perivisceral 

 cavity or its prolongations, and are apparently always con- 

 tained in processes of the lining membrane of that cavity. 

 It is not clear whether hermaphrodism is the rule or the ex- 

 ception. Thecidium, however, has been shown by Lacaze- 

 Duthiers to be dioecious ; and, according to Morse, the sexes 

 are distinct in Terebratvlina and Discina. 



The development of the Braehiopoda, notwithstanding 

 the important observations of F. Miiller, 2 Lacaze-Duthiers, 3 

 and especially of Morse, 4 stood much in need of further eluci- 

 dation (especially in regard to the earlier conditions of the 

 embryo), until quite recently, when the investigations of 

 Kowalewsky 5 filled up the hiatus in our knowledge for the 

 genera Argiope, Thecidium, Terebratula, and Terebratulina. 

 The egg becomes converted into a vesicular morula, in which 

 an alimentary sac is developed by invagination, and this sac 

 gives off, as in Sagitta, two diverticula, which become shut 



1 " Recherches pour servir a l'histoire des Bvachiopod.es." (" Journal de 

 Conch vliologie," 1860.) 



2 " Beschreibung einer Brachiopoden-Larva." (" Archiv far Anat.," 1860.) 



3 " Histoire de la Thecidee." (" Ann. d'Hist. Nat.," 1861.) 



4 " On the early stages of Terebratulina septentrionalis." (" Memoirs of the 

 Boston Society of Natural History," 1869. and the memoir already cited). 



5 Contained in a memoir, published at Moscow in 1874, for which I am in- 

 debted to the courtesy of the author. It is in Russian ; but T have been able to 

 acquaint myself with its contents, to some extent, by the aid of a friend. 



