However,  the  crayfish's  organs  are  complex  enough  for  the
needs  of  its  sluggish  life^  ^ibiii  the  more  highly  developed
muscular  and  nervous  activities  of  the  highest  backboned
animals  is  associated  more  specialization  and  complexity  of
all  organs  which  are  essential  for  the  individual  life.  The
great  significance  of  this  fact  is  impressed  upon  us  if  we
stop  to  consider  that  man  (the  highest  organism)  is  espe-
cially distinguished  by  the  complexity  of  his  nervous  organs
along  with  remarkably  perfect  muscular  functions ;  and  that
to  this  development  of  the  nervous  and  muscular  systems


868  APPLIED  BIOLOGY

we  owe  all  that  has  placed  man  so  far  above  ordinary  animal
life.  But  such  high  development  of  the  nervous  and  mus-
cular organs,  whose  activities  make  human  Hfe  worth  living,
would  have  been  impossible  without  parallel  increase  in  the
complexity  of  those  organs  (digestive,  respiratory,  circula-
tory, and  excretory)  upon  which  the  muscular  and  nervous
organs  dei)end.
312.   Molting  of  Crayfish.  —  It  is  evident  that  an  animal
inclosed  in  a  hard  external  skeleton  like  that  of  the  crayfish
cannot  grow  rapidly  while  surrounded  by  such  a  coat-of-mail.
