AND  PHYSIOLOGY  OF  THE  SPONGIAD^. 
301 
Abbreviated  bihajVIATe. — The  biliamate  spicula,  especially  the  simple  form,  are  sub- 
ject to  considerable  varieties  of  size  and  shape.  Sometimes,  as  in  abbreviated  biha- 
mate,  we  have  the  hooks  terminating  abruptly  immediately  beyond  the  proximal  curves, 
as  represented  in  Plate  XXIV.  fig.  42.  I have  found  but  very  few  specimens  of  this 
form,  and  in  no  case  in  situ ; and  I am  therefore  in  doubt  whether  it  be  an  adult  spicu- 
lum,  or  merely  a variety  arising  fi'om  an  arrest  of  development. 
Deflected  bihajmate. — When  the  hami  are  both  defiected  in  the  same  direction  at 
nearly  right  angles  to  the  plane  of  the  shaft  (Plate  XXIV.  fig.  43). 
The  variety  in  the  amount  of  curvature  at  the  middle  of  the  shaft  of  the  spiculum  is 
also  very  great,  as  represented  in  Plate  XXIV.  figs.  39,  42  and  43,  but  these  variations 
are  not  purely  accidental ; on  the  contrary,  they  are  more  or  less  constant  in  each  spe- 
cies of  sponge,  and  frequently  afibrd  good  specific  characters. 
In  the  simple  bihamate  form,  where  the  two  hami  are  curved  in  the  same  plane  and 
towards  each  other,  the  spiculum,  in  its  natural  condition,  is  usually  attached  to  the 
surface  of  the  membrane  by  the  middle  of  the  back  of  the  curved  shaft,  and  the  two 
hooks  are  projected  into  the  sarcode  at  right  angles  to  the  plane  of  the  membrane  on 
which  it  is  based.  VTien  the  hami  are  developed  reversed  or  at  right  angles  to  each 
other,  one  of  them  is  then  usually  imbedded  sideways  on  the  membrane,  and  the  other 
with  the  shaft  is  projected  from  the  plane  beneath  into  the  sarcode  at  various  degrees 
of  angle.  Or  in  the  defiected  form  the  shaft  may  be  firmly  cemented  to  the  membrane 
by  one  side,  while  the  hami  are  both  projected  upward  into  the  mass  of  sarcode.  In 
some  species  of  sponge  one  or  the  other  of  these  forms  more  especially  prevails,  but  in 
others,  as  in  Halichondria  incrustans^  Johaston,  the  simple,  reversed,  and  contort  forms 
are  indiscriminately  mixed  in  the  tissues,  and  they  occur  in  every  imaginable  form  of 
attachment  in  great  profusion,  and  accompanied  by  the  anchorate  forms  as  well. 
The  type  of  this  form  of  spiculum,  the  simple  bihamate,  is  not  peculiar  to  the  Spon- 
giadge ; it  occurs  in  a much  more  highly  organized  class,  in  a radiate  animal,  Echinus 
sphccra,  Forbes,  ‘ British  Starfishes,’  where  we  find  an  abundance  of  these  organs  disposed 
on  the  external  smTace  of  the  tubular  suckers  of  the  animal,  but  they  are  coinposed 
of  carbonate  of  hme  instead  of  silex.  I am  indebted  to  my  friend  Mr.  John  Howard 
Stewart  for  my  knowledge  of  this  interesting  fact. 
Sometimes  the  simple  forms  of  bihamate  spicula  have  the  middle  of  the  shaft  umbo- 
nate,  and  this  occurrence  is  subject  to  three  varieties : — 
Exter-umbonate. — When  the  umbo  is  on  the  middle  of  the  outer  curve  of  the  shaft 
(Plate  XXIV.  fig.  44). 
Inter-umbonate. — When  the  umbo  is  on  the  middle  of  the  inner  curve  of  the  shaft 
(Plate  XXIV.  fig.  45). 
Bi-umbonate. — When  the  middle  of  both  the  inner  and  outer  curve  of  the  shaft  have 
an  umbo  (Plate  XXIV.  fig.  46). 
Clavated  BIH.AMATE. — This  singular  form  of  bihamate  spiculum  was  found  by  Mr. 
Topping  in  a small  piece  of  sponge  from  the  coast  of  Sicily,  the  terminations  of  the 
