82 



THE ECHINODERMATA. 



$$ 79, 80. 



CHAPTER III. 



NERVOUS SYSTEM. 



§79. 



The central portion of the nervous system consists of a' ring which is 

 usually pentagonal, and surrounds the commencement of the oesophagus. 

 The main nervous branches are given off from this, and pass to the other 

 end of the body along the median line of the rays, or their corresponding 

 parts. The form of this ring is mainly due to that of the mouth ; and 

 therefore, with the renifonn mouth of Spatangus, it is unequally pentago- 

 nal.'^' Ganglia have not yet been found in it. But in Echinus and Holo- 

 thuria, the nerves passing from it have between their fibres, violet, green, 

 or red pigment granules.'-' 



§80. 



The principal nervous trunks have a longitudinal furrow, as if composed 

 of double cords, and give off from each side, during their course, branches 

 which go to the ambulacra.'^' 



With the Criuoidea, a nervous cord passes beneath the furrow formed 

 by the perisoma on the ventral surface of the arms ; this has a slight 

 swelling opposite each pinnula, to which it sends off a branch.'-' With 

 the Asteroidae, the nervous trunks which pass oft' from the oesophageal ring- 

 are lodged in the ventral furrows of the rays.'"' But in the Ophiuridae, 

 they pass in a canal, concealed by the ventral plates of the arms. The 

 five nerves, analogous to those of the Echinoidea, pass along the internal 

 surface of the ambulacral plates, between the vesicles, even to the centre 

 of the dorsal region. In Echinus, there are, moreover, special nerves 

 directly from the oesophageal ring, for the organs of mastication and 

 digestive canal.'*' In Holothuria, this ring is situated directly on the 

 anterior border of the osseous circle, and sends off five nerves which pass 

 along the median line of the longitudinal muscles, even to the end of the 

 body ;'^ it sends off also special nerves to the oral tentacles."" 



1 Krohn (Mullcr's Arch 1841, p. 8, Taf. I. fig. 

 3,4). 



2 Krohn, loc. cit. 



1 Krohn, ibid. p. 4, 10. 



2 Miiller (Abliandl. d. Berl. Alcad. loc. Cit. p. 

 233, Taf. IV. fig. 11, i. ; Taf. V. fig. 16). 



3 The nervous system of the Asteroidae was first 

 clearly shown by Tiedemann (loc. cit. p. 62, 

 Taf. IX. and Meckel's Deutsch. Archiv. I. 1815, 

 p. 69, Taf. HI. fig. 1). This anatomist, like 

 Krohn (loc. cit. p. 4), did not perceive the ganglia 

 of the oesophageal ring, observed by Wagner 

 (Vcrgleich. Anat. 1834, p. 372). 



The ganglia and nerves that Spix (Ann. du 

 Mus. d'Hist. Nat. Xni. 1809, p. 439, PI. XXXII. 

 fig. 3, 6) and Konrad (De Asteriarmn faln-ica 

 dissert. 1814, p. 13, fig. 3, o.) affirm to have seen 

 on the internal (dorsal) surface, opposite the ventral 



furrows of the articulations of the rays, in Aster- 

 acanthion rubf.ns, and glacialis, are probably 

 only tendinous fibres. 



i Krohn, who has studied the nervous system 

 of Echinus and Spatangus, has traced the fila- 

 ments given off from the main trunks, across the 

 ambulacral pores, to the suckers of the ambulacra. 

 See also Valentin''s figm-es of this system, in 

 Echinus (Mouogr. loc. cit. p. 98, PI. VIII. IX.). 



5 The oesophageal ring of Holothuria, observed 

 by Krohn {MUller's Arch. 1841, p. 9, Taf. I. fig. 

 5), sends off its prmcipal nerves across the fissures 

 of the dentations of the five great pieces of the 

 osseous rings. Their lateral filaments, going to the 

 ambulacral vesicles, are so fine that Krohn could 

 scarcely find them. 



6 Grant, loc. cit. p. 184.* 



* [ § 80, note 6.] Miiller has furnished some Holothurioidea ; see Arch. 1850, p. 226. He 

 valuable contributions on the nervous system of the makes tlais statement, which 13 worthy of remem- 



