ANATOMY AND CLASSIFICATION OP THE ARENICOLID^. 483 



we have been able to trace a process of tlie lateral ^' giant- 

 fibres " into the spinal nerves (p. 491) , but this does not occur, 

 so far as we ascertained, in the case of every pair of nerves. 

 This process is generally distinguishable from the other fibres 

 by its sheath and delicately fibrillar contents. 



In addition to the motor and sensory fibres, the nerves also 

 contain stellate cells, generally somewhat shrunken, so that a 

 space separates thebody of the cellfrom the surrounding fibrous 

 tissue. The nucleus contains a chromatin network, and the 

 protoplasm is differentiated into fine sinuous fibrilltB staining 

 deeply with iron haamatoxylin. These cells are doubtless 

 of neuroglial nature. We have also noticed a few scattered 

 cells which are apparently ganglionic. They occur chiefly 

 at the commencement of the nerve just after it leaves the 

 cord, but owing to the failure of the elements to respond 

 to methylene blue we are unable to trace the processes of 

 these cells, or to distinguish satisfactorily the motor from 

 the sensory fibres of the nerves. 



IV. Giant-cells and Giant-fibres. 



A. Giant-fibres. 



Giant-fibres occur in all species of Arenicola except A. 

 Claparedii, but we have studied them with most care in A. 

 Grubii. In a piece of the fresh cord of this species they 

 appear as tubes ("l — "Smm. diam.) with a distinctive wall and 

 homogeneous contents. Anteriorly there is a single median 

 fibre ; in segments five to ten there are two ; in the central 

 region of the body there are generally three ; and posteriorly 

 only one. From the inner surface of the wall of the fibre, 

 shreds or granular tags project into the interior, but oVing 

 to the opacity of the enveloping neurilemma and the granu- 

 lar cells of the peritoneum, it is impossible to make an ex- 

 haustive examination of the giant-fibres by the simple 

 inspection of the fresh cord. Some evidence of the nature 

 of these fibres is afforded by their behaviour towards re- 

 agents. If a 2 per cent, solution of osmic acid be added to 

 unaltered pieces of fibre they become brownish, but not black, 



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