No. I.] THE METAMERISM OF NEPHELIS. 41 



Structure of the characteristic features of the cell remain 

 unchanged from those of the normal mid-body cell. 



Median Nerve Cells. 



Within the fibrous body portion of the normal ganglion, near 

 the median plane, lie two "median nerve cells," one slightly 

 anterior, the other posterior, to the center. They are found in 

 all the Hirudinea and have been described by several authors. 

 Retzius (12) and Biedermann (13) show them in their figures of 

 Hirudo obtained by methylen blue, and they continue to appear 

 forward in the sub-oesophageal ganglia. In my analysis of the 

 " brain " I shall speak of these in detail. I have also found 

 them in the anterior neuromeres of the "anal ganglia," but I 

 am not able to say whether they are present in the posterior 

 neuromeres or not. 



The Fibrous Portion. 



The fibrous part of the ganglion occupies the axial portion 

 and, rnacroscopically, appears to consist of thickenings of the 

 two connectives that afterwards fuse. It is perforated by two 

 small holes which lie close together on either side of the median 

 plane at the level of the anterior nerves. These perforations 

 persist in the fused portions of the nerve chain and afford good 

 evidence of the fusion of originally separate neuromeres. 



According to Biedermann (/. c) and Retzius (/. c.), this fibrous 

 part is made up of fibers from three different sources : (i) from 

 the connectives, part of which continue through the ganglion ; 

 (2) the efferent fibers from the neurones, which fill the six cap- 

 sules of the ganglion ; and (3) the afferent fibers, which are 

 the central termini of neurones whose trophic centers lie out- 

 side of the ganglia. 



The first two sources are readily demonstrated, but Retzius 

 failed to find the source of all the fibers in the third set. His 



1- G. Retzius : Biologische Untersuchungen. Neue Folge, 2. Stockholm. 1890. 



1' W. Biedermann : "Ueber den Ursprungund die Endigungsweise der Nerven 

 in den Ganglien wirbelloser Tliiere." Jenaische Zeitschrift fiir Naturwiss., Bd. x.xv, 

 1891. 



