598 THE ANIMAL KINGDOM. 



each ganglion being widely separate and united by transverse commissures. 

 The longitudinal commissures between successive pairs of ganglia are 

 separated in PhyUodocidae, but as a rule they are contained within a 

 common sheath. The nerves given off laterally from the ganglia to the 

 parapodia sometimes possess a small ganglion where they branch. The 

 ganglion cells may be spread over the whole extent of the cords, as in 

 Lumbriculidae and Lumbricidae among Oligochaeta, and in the anterior 

 part of the cords in some Polychaeta, e. g. Nephthys^ but they are usually 

 aggregated in the ganglia. The stomato-gastric system is present in all 

 Oligochaeta (p* 211) and most Polychaeta. In the latter it originates from 

 the cerebral ganglia (e. g. Eunicidae, Serpulidae), from the same ganglia and 

 the oesophageal commissures as well (e. g. Nephthys, Phyllodoce}, or from 

 the commissures alone (e. g. Ophelia}. This last mode of origin obtains in 

 all Oligochaeta. It is best developed in those Polychaeta which possess a 

 muscular pharynx 1 . Supporting or skeletal structures, the so-called ' giant- 

 fibres ' or ' neurochord,' are found in nearly all Oligochaeta and many 

 Polychaeta on the dorsal aspect of the ventral cords (p. 211). For the 

 lateral ganglionic cords of Oligochaeta, see p. just cited. 



Organs of special sense are found in the form of ciliated grooves, 

 tactile cells or bodies, eyes, and otocysts. Ciliated grooves occur on the 

 prostomium of the Oligochaete Aeolosoma, and many Polychaeta, and are 

 supplied by nerves from the cerebral ganglia. Their function is unknown ; 

 it is possibly olfactory 2 . Tactile hypodermic cells furnished with external 

 setae, long or short, and connected basally with a nerve filament, derived 

 either from a nerve or a ganglion cell are probably widely distributed. For 

 their occurrence in Oligochaeta, see p. 311. In Polychaeta they occur aggre- 

 gated on papillae of the antennae and the cirri, and of the elytra of 

 Aphroditidae, and as special organs in the ' goblet ' organs of the Capitellidae 

 where they are scattered over the proboscis, prostomium, and thorax ; or 

 in Polyophthalmus as a single pair in connection with the ciliated grooves ; 

 and as * lateral ' organs, one between each noto- and neuro-podium in the 

 Capitellidae, or the corresponding bundles of setae of Polyophthalmus 

 The nerves to the goblet and lateral organs end in ganglia. Eyes are 

 absent in all Oligochaeta except some Naidomorpha. They consist in Nais 

 proboscidea of a few large cells covered on one side by small pigmented cells. 

 They may be absent also in some Polychaeta and rudimentary in others, 

 and lodged either within, or in close apposition with, the cerebral ganglia. 

 As a rule they are confined to the prostomium, but among Tubicola 



1 It forms in Eunicidae a sub-pharyngeal ganglionated cord and an oesophageal ring. It has 

 in Nephthydae and Phyllodocidae long roots, which end in a ring of minute ganglia. Full details are 

 given by Pruvot. 



2 Ganglion cells are found in connection with the grooves in Polyophthalmus. A pair of grooves 

 is found on each segment of the body in Aricia. 





