FUNCTIONS  OF  THE  HAIES  OF  THE  CEUSTACEA. 
897 
be  made  in  such  a part  in  the  plane  of  the  hairs,  the  canals  leading  to  them  will  be 
often  found  to  be  nearly  as  large  as  the  base  of  the  hairs  to  which  the  canals  correspond. 
They  are  lined  by  a thick,  transparent,  chitinous  membrane,  within  which  may  be  seen 
the  outer  layer  of  the  internal  integument.  The  chitinous  lining,  after  investing  the 
cup-shaped  ca\ity  which  receives  the  hair,  becomes  so  firmly  connected  with  the  bulb 
of  the  hair  itself,  that  on  tearing  out  the  hair,  which  cannot  be  done  without  force,  the 
sheath  will  often  be  dragged  out  with  it. 
The  cells  and  other  elements  of  the  spongy  middle  layer  of  the  internal  integument 
fill  up  the  canal,  and  pass  on  through  the  opening  at  the  root  of  the  hau'  into  the  hair- 
canals. 
In  every  part  in  which  the  hahs  have  been  examined,  their  connexion  with  the  vascu- 
lar internal  membrane  has  been  observed.  But  where  the  shell  is  very  thick,  as  in  the 
claw  of  the  Lobster,  some  manipulation  is  necessary  to  its  clear  demonstration.  If  fine 
sections  be  made  of  a claw  which  has  been  softened  in  dilute  nitric  acid,  the  canals  of 
the  shell  will  be  seen  to  terminate  in  cup-shaped  cavities,  which  receive  the  hair-bulbs 
and  are  continuous  with  the  ham-tubes.  The  shell  canals  are  comparatively  fine,  more 
resembling  coarse  dentinal  tubes ; but  they  are  lined  by  a sheath,  and  have  contents 
prolonged  from  the  vascular  layer.  If  the  plan  be  adopted  by  which  Mr.  Tomes  demon- 
strated the  presence  of  the  dentinal  fibres*,  viz.  tearing  the  section  across  the  direction 
of  the  canals,  them  sheaths  mil  be  dragged  out  in  the  form  of  tubes,  from  which  the 
contents  have  sometimes  escaped,  so  that  only  air  is  contained  in  them,  though  generally 
their  contents  may  be  still  seen  in  them  (fig.  2 a).  The  relations  of  the  contents  of  the 
tubes  to  the  internal  integument  may  be  shown  by  tearing  away  the  latter  from  the 
shell,  when  the  contents  will  often  be  drawn  out  of  the  canals ; and  it  may  be  seen  that 
they  are  prolongations  of  the  outer  layer  of  the  integument,  enclosing  the  elements  of  the 
vascular  layer  within  their  tubular  cavities  (fig.  2 c). 
The  uniform  and  intimate  connexion  of  the  hairs  with  the  living  structures  within 
the  shell  indicates  that  they  are  not  mere  appendages  to  it,  like  the  tubercles,  but  that^ 
they  have  some  function  distinct  from  it,  and  associated  rather  with  the  deeper-seated 
nervous  and  vascular  structures.  The  most  natural  conclusion  appears  to  be,  that,  like 
the  whiskers  of  the  Seal  or  of  the  fehne  Carnivora,  and  indeed  like  the  hams  of  the  Mam- 
malia generally,  they  are  a means  of  transmitting  sensation  from  without  to  the  sensitive 
parts  within.  It  is  generally  assumed  that  the  greater  part  of  the  body  of  crustaceans 
is  destitute  of  sensibility,  though  the  antennse  and  oral  appendages  have  been  known  to 
possess  it  to  a great  extent.  It  is  not  easy  to  conceive,  however,  that  the  dense  calcareous 
coverings  of  these  parts  could  be  the  medium  of  communicating  any  delicate  impressions, 
such  as  are  evidently  conveyed  by  the  antennge.  There  is  no  difficulty  in  understanding 
this,  on  the  supposition  that  the  hairs  which  surround  each  joint  of  the  antennse,  and  each 
border  of  the  oral  appendages,  are  in  dmect  relation  with  a sensitive  pulp  within,  and 
that  the  impressions  made  upon  their  base  would  be  at  once  communicated  to  this  pulp. 
* Philosopliical  Transactions,  1856. 
6 A 2 
