NERVOUS SYSTEM. INVERTEBRATA. 15 



There are no giant fibres in the connectives. The ventral 

 cord is surrounded by a double neurilemma-sheath 

 (i.) an outer sheath that loosely envelopes the cord and 

 extends for some distance along the lateral nerves : it forms 

 the outer wall of the perineural blood-vessel ; (ii.) an 

 inner sheath, that closely invests the cord and binds the 

 three connectives into a single strand. Offshoots from 



Fig. 10. 





Transverse section through a Ventral-chain Ganglion of Hirudo medicinalis. 



X 125. 



C.M. Central medulla. G.C. Ganglion-cells. G.GC. Giant ganglion-cells. 



this inner sheath penetrate amongst the fibres of the con- 

 nectives and dip into the substance of the ganglia, where 

 they separate the ganglion-cells into three definite groups 

 and form an investment to the central medulla. 



Both sheaths contain many muscle-fibres. 



The outer neurilemma-sheath has been removed and 

 black paper placed beneath the nerve-cord. 0. C. 1295 B. 



Leuckart, Die Parasiten des Menschen, Bd. i. 1894, p. 579. 



GEPHYEEA. v 



D. 10. The anterior and posterior parts of the body- walls of a 

 Gephyrean (Sipunculus nudus) showing the nervous system. 

 The cerebral ganglion is a rounded mass with slight lateral 

 swellings, situated upon the dorsal surface of the oesophagus 

 at the base of the tentacles. Nerves for the tentacles arise 



