39 
nucleus  is  situated  („ glandes  finement  granulées"  de  Cebfontaine).  More 
over  the  clitellum  appears  also  to  contain  unchanged  hypodermis-cells, 
the  nuclei  of  which  lie  in  a  continuous  row,  quite  near  the  surface 
(Pl.  II.  fig.  17).  The  glandular  thickening  of  the  clitellum  usually  com- 
mences with  the  XVIIIth  segment,  but  on  transverse  sections  the 
first  glandular  cells  are  already  visible  in  the  Xlllth  segment;  there- 
fore the  clitellum  appears  to  extend  itself  much  more  forward,  as 
also  visible  externally  in  some  specimens.  The  glandular  layer  has 
nearly  the  same  thickness  over  the  whole  periphery  of  the  clitellum, 
but  the  distribution  of  its  glandular  elements  is  not  the  same  every 
where  (pl.  II.  fig.  18).  At  the  dorsal  side  there  is  a  couple  of  peripherical 
layers  of  coarsely  granular,  tubular  cells,  followed  interiorly  by  six 
to  eight  layers  of  finely  granular  clubshaped  cells;  at  the  ventral 
side  the  same  arrangement  is  to  be  seen  only  in  its  median  portion, 
between  the  ventral  bristles.  The  lateral  portion  of  the  ventral  side 
only  consists  of  coarsely  granular  cells. 
A  similar  arrangement  is  stated  by  Ceefontaine  to  exist  in  the 
clitellum  of  Lumbricus  terrestris;  in  this  species  he  found  also  in  the 
tubercula  pubertatis  modified  club-shaped  cells,  containing  a  more 
coarsely  granular  substance. 
The  diagram  (fig.  18)  shows  that  the  muscular  layer  does  not  enter 
into  the  substance  of  the  ridge;  its  internal  substance  consists  of  a 
network  of  fine  connective  fibres  and  ramified  cells.  Several  blood- 
vessels are  also  to  be  found  therein. 
External  openings  of  the  genital  organs  or  of  the  nephridia  are  not 
visible;  dorsal  pores  are  absent. 
In  the  vicinity  of  the  clitellum  curious  oval  grooves,  with  a  dark 
protuberance  in  their  centre,  are  visible  (pubertätsgrübchen 
Michaelsen).  Neither  the  arrangement  nor  the  number  of  these  pu- 
berty-grooves is  constant.  In  the  XXIIth  segment  they  are  usually  to 
be  found  on  each  side  in  the  series  of  the  external  ventral  bristles, 
in  segment  XXIII,  XIX  and  XX  however  in  the  series  of  the  infe- 
rior dorsal  bristles;  they  can  also  be  met  with  in  segment  XXXII 
and  XXXIV.  These  grooves  often  are  situated  assymetrically  f.  i.  at  the 
left  side  of  the  body  in  segment  XVIII  and  XIX ,  at  the  right  side  in 
segment  XIX  and  XX.  In  transverse  sections  (fig.  17)  they  appear  to 
be  cup-shaped  organs,  which  are  very  obvious  by  their  vitreous  ap- 
pearance and  lie  totally  embedded  in  the  glandular  layer  of  the  clitel- 
