COMMON EARTHWORM. 311 



The nerves originating from the cerebral ganglia usually break up into a plexus, 

 in which ganglion cells are interpolated. The network is especially well-developed 

 and visible in Tubifex, and in Limnodrilus and Anachaeta its ganglion cells are 

 aggregated into special ganglia. 



The first ganglion of the ventral chain generally gives off many nerves ; the 

 following ganglia a pair of nerves on either side. A single septal nerve arises also 

 on each side, between successive ganglionic enlargements, and is distributed to a 

 septum. 



A pharyngeal plexus or ' Vagus ' system is derived in most, probably in all, Oligo- 

 chaeta from the oesophageal commissures. In some of the lower Oligochaeta there 

 are distinct pharyngeal ganglia. In the Earthworm the system consists usually of 

 a more or less elongated mass lying on either side of the pharynx, continuous with 

 a rich plexus, the fibres of which are ultimately lost among the muscular structures. 

 Ley dig and Vignal state that ganglion cells are found in all parts of this plexus ; 

 Claparede, on the contrary, that none occur in the plexus so far as it is visible to the 

 naked eye, but that they are present on the finer branches among the muscular 

 bundles. 



A cord of ganglion cells in continuity anteriorly with the cerebral ganglia runs 

 down each side of the body in most Oligochaeta. It is especially easy to see in 

 JVatSj and is contained in the hypodermis. It is said to supply the muscles of the 

 head, sacs of the setae, and the nephridial apertures ; and is, perhaps, in connection 

 with the ganglion cells, which have been found in the walls of the digestive tract in 

 some instances. A zone of ganglion cells encircles each somite in the Naid 

 Slavina appendiculata, is connected to the lateral cords, and supplies the tactile 

 eminences (infra]. In Lumbricidae the lateral cords are to be detected clearly in 

 the young posterior somites, but they are resolved in the older anterior somites 

 into scattered cells. 



As to organs of special sense. Oligochaeta never possess otocysts. Eyes are 

 found only in some Naidomorpha \ supposed gustatory organs in the pharynx of 

 Enchytraeidae and Limnodrilus ; and olfactory (?) organs as a couple of ciliated pits 

 on the head in Aeolosoma, Ctenodrilus, Parthenope. The last-named structures 

 occur also in some Polychaeta. Tactile organs, however, are commonly distributed, 

 and in various forms; as (i) a hypodermis cell furnished with an external tactile 

 seta, and continued basally into a nerve fibril, in its turn often connected to a 

 ganglion cell, found in numbers on the prostomium of Aeolosoma, Chaetogastridae, 

 and Naidomorpha ; (2) tactile papillae, in which the hypodermis cell is protrusible, 

 and furnished with short setae ( Chaetogastridae] ; (3) tactile eminences, apparently 

 composed of aggregations of tactile hypodermis cells, arranged fifteen to twenty in 

 number in a zone on each somite of Slavina appendiculata ; and (4) goblet bodies, 

 or aggregations of very delicate hypodermis cells provided with sense-hairs, found 

 most plentifully on the prostomium and buccal somite of Lumbricidae, more 

 sparingly on the anterior somites of the body, but especially round the setae. A 

 single goblet body is found also on each side of the somites of the Lumbriculidae, 

 seated on the lateral cord of ganglion cells. They become enlarged on the clitellum 

 of the Lumbriculid Rhynchelmis, where gland cells occur among the sense cells. 

 See on the subject Vejdovsky, op. cit. pp. 96-100. 



On the dorsal aspect of the nerve-cord there are to be found three remarkable 



P 3 



