NERVOUS SYSTEM. INVERTEBRATA. 5 



HOLOTHUKOIDEA. 



D. 3. A Sea-cucumber (Holothtiria nigra) with the nervous system 

 shown by the removal of the bivial body-walls. 



The oral ring of the superficial system surrounds the 

 mouth under cover of the calcareous ring and in close 

 contact with the oral integument. The five radial cords 

 given off from it pass beneath the radial pieces of the cal- 

 careous ring and along the radii; they are separated from 

 the deep surface of the integument by an epineural canal. 



The deep oral system (not visible in the specimen) is 

 confined to the inner surface of the radial cords, and is 

 separated from the muscles of the body-wall by the pseud- 

 hsemal, blood-vascular, and ambulacral vessels. The re- 

 spective share taken by the two systems in the innervation 

 of the body has not been satisfactorily determined. There 

 is no apical system. Black paper has been inserted beneath 

 the oral ring at the points of origin o the radial cords and 

 in various places beneath the cords. The calcareous ring 

 has been removed. 0. C. A 1292 b. 



Ludwig, Bronn's Thier-reich, Bd. ii. Abth. 3, 1889- 

 1892, p. 64. 



ANNELIDA. 



Retzius, Biol. Untersuch., N.F. Bd. ii. p. 1, iii. p. 1, iv. p. 1, 

 vii. p. 6, ix. p. 83, 1891-1900. 



The central nervous system is bilaterally symmetrical ; it 

 consists typically of a pair of prseoral (cerebral) ganglia situated 

 in the prostomium, and of a series of post-oral ganglia, arranged 

 segmentally in pairs along the ventral mid-line of the body and 

 united together by transverse commissures and longitudinal 

 connectives. This ventral chain of ganglia shows great diversity 

 in the degree of concentration of its parts in a transverse direc- 

 tion. Longitudinal concentration is rare and never extreme. 

 Frequently the segmental ganglionation is absent, and the 

 chain is then represented by a fibrous cord with a continuous 

 layer of ganglion-cells on its ventral surface. The size and 

 complexity of structure of the cerebral ganglia depend entirely 

 upon the degree o development of the cephalic sense-organs. 

 Apart from particular functions due to their connection with 



