82 THE ECHINODERMATA. '^s'^. 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 brauches 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 reniform mouth of Spatangus, it is unecjually pentago- 

 nal. *'' Ganglia have not yet been found in it. But in Echhms 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 Grinoklea, a nervous cord passes beneath the furrow formed 

 by tne 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 off 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 Echhms, 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 cii'cle, 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 (Miiller^s Arch. ISil, p. 8, Taf. I. fig. furrows of the articulations of the rays, in Aster- 

 3,4). acanthion rubens, and glacialis, are probably 



2 Krohn, loc. cit. only tendinous fibres. 



1 Krohn, il)id. p. 4, 10. 4 Krohn, who has studied the nervous system 



2 Midler (Abhandl. d. Berl. Akad. loo. cit. p. of Echinus and Spatangus, has traced the fila- 

 233, Taf. rV. fig. 11, i. ; Taf. V. fig. 16). ments given off from the main trunks, across the 



3 The nervous system of the Asteroidae was first ambulacral pores, to the suckers of the ambulacra, 

 clearly shown l)y Tiedcmann (loc. cit. p. 62, See also ViilentuVs figures of tliis system, in 

 Taf. iX. and MeckeVs Deutsch. Archiv. I. 1815, Echinus (Jlouogr. loc. cit. p. 98, PI. VIII. IX.). 

 p. 69, Taf. III. fig. 1). This anatomist, like 5 The (Esophageal ring of Holothuria, observed 

 Krohn (loc. cit. p. 4), did not perceive the ganglia by Krohn (Mutle,r''s Arch. 1841, p. 9, Taf. I. fig. 

 of the oesophageal ring, observed by IVagner 5), sends off its principal nerves across the fissures 

 (Vergleich. Anat. 1834, p. 372). of the dentations of the five great pieces of the 



The ganglia and nerves tliat Spix (Ann. du osseous rings. Their lateral filaments, going to the 



Mus. d'Hist. Nat. XIII. 1809, p. 439, PI. XXXII. ambulacral vesicles, are so fine that Krohn could 



fig. 3, G) and Konrad (De Asteriarum fabric'a scarcely find them, 



dissert. 1814, p. 13, fig. 3, o.) altirm to have seen G Grant, loc. cit. p. 184.* 

 on the internal (dorsal) surface, opposite the ventral 



* [ § 80, note 6.] Milller has furnished some Ilolothurio'idea ; see Arch. 1850, p. 226. He 

 valuable contributions on the nervous system of the makes this statement, which is worthy of remem- 



