GANGLIONIC NERVOUS SYSTEM 



519 



is seen in the simplest unsegmented worms, such as 

 some Turbellarians, wherein this single ganglion is 

 responsible for relaying sensory messages and co- 

 ordinating motor responses (Fig. 681). In other Turbel- 

 larians, two to eight nerve -cords run posteriorly from 

 the ganglion (Fig. 682) ; each contains nerve cells, not 

 yet grouped into ganglia, and gathers afferent fibres ; 

 and so long as these are left intact — but only so long — 

 spontaneous movement and coordinated responses 

 persist. 1 When the rudimentary ocelli are few they are 

 grouped on the dorsal aspect of the anterior end and 

 the nerve fibres run directly into the cerebral ganglion ; 

 when they are many and diffusely scattered, they enter 

 the peripheral nerve-net. The former arrangement is 

 also seen in the larvse of some Insects (Fig. 683). 



Again, the simple system of a single ganglion controlling 

 a peripheral mechanism is seen among the primitive Proto- 



chordates in the Tunicates ; when the ganglion is removed the Ascidian may slowly 

 develop reflexes confined to a single siphon when stimulated, but all inter-siphonal 

 responses and general coordination are lost (Kinoshita, 1910 ; Day, 1919 ; Prosser, 

 1946). 



In SEGMENTED WORMS (annelids), however, the nerve-cells are grouped 

 into ganglia, each subserving the receptor-effector mechanism of its own and 

 often adjacent segments. The simplest form of such a system is seen in 

 Oligochaetes such as the earthworm, Lumbricus (Fig. 684). Situated dorsally 

 in the third segment are two cerebral ganglia from which emerge two 

 nerve-cords ; initially these form a ring around the pharjaix beneath which 



Fig. 683.— The Eye 

 OF THE Larva of 



AciLlUS. 



The fibre-like pro- 

 longations of the light- 

 sensitive cells, R, go 

 directly into the cere- 

 bral ganglion, G (after 

 Gaskell). 



Fig. 684. — Nervous System of a Segmented Worm. 



Transverse section through the earthworm. S.E., surface epithelium ; CM., 

 circular muscles ; L.M., longitudinal muscle ; S^, sensory cell the fibre from which 

 terminates directly in the subepidermal nerve-net ; S-, sensory cell the fibre from 

 which goes to a segmental ganglion to merge in the neurojjile ; A, association neurone, 

 the processes from which do not leave the central nervous system but run mainly up 

 and down the ganglionic chain (perpendicular to the page) ; M, two motor neurones, 

 the dendrites of which contribute to the neuropile ; N, neuropile, comiDosed of pro- 

 cesses of the three types of cell — sensory, motor and associative. 



1 Bardeen (1901) in Planarians 

 Rietschel (19.35) in Cestodes. 



Eggers (1924) and Friedrich (1932) in Nemertines 



