122  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STEUCTrEE  AND  HOMOLOGY 
nexion  with  the  ingoing  hmb,  in  Arenicola  with  the  body  of  the  organ.  This  circum- 
stance with  still  greater  emphasis  points  to  the  ingoing  limb  (/  m,  B)  as  the  seat  of 
the  ovogenesis ; indeed  the  enthe  dark  half  of  the  organ  (m/)  may  be  correctly  called 
an  ovary.  The  limbs  of  this  organ  in  Terehella  have  precisely  the  same  anatomical  rela- 
tions as  in  Arenicola.  The  ingoing  leg  {fm,  B)  {ffj,  A)  commences  in  an  external 
orifice  ((^B,  cA),  and  therefore  carries  water.,  as  in  Aremcola.  This  water  traverses  the 
organ  as  indicated  by  the  arrows  at  A,  fig.  12. 
The  outgoing  limb  {k  c,  A)  is  formed  after  the  model  of  the  con-esponding  part  in 
Arenicola.  The  main  channel  opens  externally  (at  c).  A secondary  tubular  process  (e) 
leads  into  the  general  cavity ; it  serves  to  convey  into  the  latter  the  ova  or  sperm-cells, 
as  they  are  conducted  to  this  spot  by  the  ciliary  epithehum  which  lines  the  interior  of 
the  organ  (A). 
It  will  be  seen  that  one  half  (fm,  B,  fig.  12)  of  the  segmental  organ  in  Terehella  is 
highly  vascular  and  glandular,  and  that  the  other  half  [n  n,  B)  is  simply  excretory  and 
membranous.  This  distinction  marks  also  the  offices  of  these  halves  respectively.  The 
peculiarly  evolved  vascular  system  is  connected  only  with  the  ovarian  or  spermatic  half, 
the  other  half  being  simply  the  oviduct  or  vas  deferens,  according  to  the  sex. 
In  the  male  and  female  Terehella  the  segmental  organs  are  precisely  the  same  in  num- 
ber, size  and  structure.  It  is  only  possible  to  distinguish  the  sex  by  the  microscopic 
examination  of  the  morphotic  elements  contained  in  the  segmental  organs  and  in  the 
general  cavity  of  the  body.  In  the  female  they  are  ova,  in  the  male  they  are  sperm- 
cells,  or  spermatozoa  in  both  situations. 
The  general  cavity  in  Terehella,  as  in  Arenicola,  acts  as  a reservoir  in  which  the  gene- 
rative products  complete  the  last  stage  of  their  development.  In  the  female  it  is  a 
vitellarium,  in  the  male  a vesicula  seminalis. 
The  segmental  organs  in  Terehella  differ  in  number  in  different  species ; in  none  do 
they  differ  in  minute  structure.  In  T.  nehulosa  this  organ  is  segmentally  repeated  sixteen 
times ; in  T.  conchilegia  only  six  times  (in  other  words,  it  is  only  present,  in  pairs,  in  six 
segments) ; in  T.  multisetosa  twenty-four  times ; in  T.  cirrata  about  eighteen  times ; in 
T.  jparmla  only  three  times.  In  all  it  is  constructed  on  the  type  of  that  of  Arenicola. 
Between  the  genus  Terehella  and  that  of  Arenicola,  as  regards  the  reproductive  system, 
there  prevails  ajgparently  one  striking  difference.  In  the  former  the  anatomist  discovers 
a large  glandular  mass,  extending  from  the  head  along  the  median  line  to  some  distance 
in  the  direction  of  the  tail.  This  gland-structure  coincides  internally  wfith  the  smooth, 
foot-like,  dense  tegumentary  structure  observable  in  every  species  of  Terehella  on  the 
thoracic  half  of  the  abdominal  aspect  of  the  body  externally. 
It  has  been  described,  by  Cuviee,  Milne-Edwaeds,  De  Quateefages,  Geube,  Stannius 
and  others,  as  the  testes. 
At  one  time  the  author  of  this  paper  also  put  upon  these  parts  a similar  construction ; 
he  has  however  since  convinced  himself  that  they  have  nothing  to  do  with  the  system 
of  the  testes,  nor  do  they  aid  in  any  way  in  the  process  of  reproduction. 
