XVIII. ON THE SPINAL NERVES OF AMPHioxus 1 . 



IN an interesting memoir devoted to the elucidation of a 

 series of points in the anatomy and development of the Ver- 

 tebrata, Schneider 2 has described what he believes to be motor 

 nerves in Amphioxus, which spring from the anterior side of the 

 spinal cord. According to Schneider these nerves have been 

 overlooked by all previous observers except Stieda. 



I 3 myself attempted to shew some time ago that anterior 

 roots were absent in Amphioxus ; and in some speculations on 

 the cranial nerves, I employed this peculiarity of the nervous 

 system of Amphioxus to support a view that Vertebrata were 

 primitively provided only with nerves of mixed function springing 

 from the posterior side of the spinal cord. Under these circum- 

 stances, Schneider's statement naturally attracted my attention, 

 and I have made some efforts to satisfy myself as to its accuracy. 

 The nerves, as he describes them, are very peculiar. They arise 

 from a number of distinct roots in the hinder third of each 

 segment. They form a flat bundle, of which part passes up- 

 wards and part downwards. When they meet the muscles they 

 bend backwards, and fuse with the free borders of the muscle- 

 plates. The fibres, which at first sight appear to form the nerve, 

 are, however, transversely striated, and are regarded by Schneider 

 as muscles ; and he holds that each muscle-plate sends a process 

 to the edge of the spinal cord, which there receives its innerva- 

 tion. A considerable body of evidence is requisite to justify a 

 belief in the existence of such very extraordinary and un- 

 paralleled motor nerves ; and for my part I cannot say that 

 Schneider's observations are convincing to me. I have attempted 

 to repeat his observations, employing the methods he describes. 



1 From the Quarterly Journal of Microscopical Science, Vol. XX. 1 880. 



2 Beitrdge z. Anat. u. Entwick. d. Wirbelthiere, Berlin, 1879. 



s " On the Spinal Nerves of Amphioxus," Journ. of Anat. ami Phvs. Vol. X. 1876. 

 [This edition, No. IX. p. 197.] 



