AND  PHTSIOLOaT  OF  THE  SPONGIAD^. 
295 
one-tenth  of  that  of  the  ray  at  any  part,  while  that  of  the  apex  of  the  shaft  occupies  about 
nine-tenths  of  its  diameter  at  that  point ; while  the  defensive  points  are  thus  strength- 
ened as  much  as  possible  to  adapt  them  to  their  especial  office,  the  shaft  is  so  con- 
structed as  to  possess  the  greatest  amount  of  combined  strength  and  flexibility.  Plate 
XXIV.  fig.  11  represents  the  head  of  a ternate  spiculum  thus  charred.  In  less-deve- 
loped spicula  we  often  find  the  central  cavity  of  the  ternate  ray  comparatively  large  and 
gradually  decreasing  to  the  apex,  and  in  older  spicula  than  that  first  described,  the 
central  caVty  of  the  shaft  is  sometimes  comparatively  small  near  its  apex ; but  if  so,  we 
generally  find  that  it  expands  rapidly  as  we  proceed  lower,  so  that  in  this  case  it  is  only  a 
transference  of  the  elasticity  to  a lower  portion  of  the  shaft.  Plate  XXIV.  fig.  8 exhibits 
the  earliest  stage  of  development  of  the  porrecto-ternate  head  of  the  spiculum ; fig.  9 a 
more  advanced  stage  of  growth ; and  fig.  10  a mature  and  fully  developed  spiculum. 
Fusifoemi-eecuevo-teeivate  spiculum. — This  form  of  defensive  spiculum  occasionally 
accompanies  the  porrecto-ternate  ones  of  the  defensive  fasciculi  of  Tethea  cranium.  The 
length  and  proportions  of  the  shaft  of  the  former  is  very  much  the  same  as  those  of  the 
latter.  The  recurvate  apex  of  this  spiculum  undergoes  a progressive  development,  which 
does  not  appear  to  commence  until  after  a great  extent  of  the  length  of  the  slender 
flexible  shaft  has  been  produced,  when  an  enlargement  of  the  apex  of  the  shaft  takes 
place,  and  the  rudiments  of  the  stout  recurvate  radii  appear  as  represented  by  fig.  12, 
Plate  XXIV.,  and  between  this  and  the  fully-developed  form,  fig.  13,  all  the  intermediate 
gradations  of  development  may  be  observed  among  the  spicula  of  young  specimens  of 
the  sponge.  The  two  figures  are  drawm  by  the  same  power,  260  linear,  and  the  difference 
in  size  between  the  young  and  the  fully-developed  spiculum  is  very  remarkable. 
In  the  calcareous  sponges,  Grantia  of  FuEMUsm,  we  find  the  defensive  spicula  which 
are  projected  into  the  great  cloacal  caVties  emanating  from  the  basal  junction  of  the 
radii  of  the  ordinaiy  triradiate  spicula  of  the  skeleton,  and  they  are  frequently  of  a 
different  form  to  that  of  the  radii  of  the  skeleton  spicula,  and  in  most  of  the  sponges  of 
this  class  they  form  very  effective  specific  characters.  The  production  of  a spicular  ray 
from  the  centre  of  the  ordinary  triradiate  skeleton  spiculum  often  seems  to  cause  an 
extra  development  of  one  or  more  of  the  radii  of  the  latter,  and  occasionally  the  basal 
radii  are  bent  or  distorted  in  a manner  rarely  observed  in  the  simple  skeleton  spicula. 
Spiculated  equiangulated  teieadiate. — When  the  spicular  ray  is  of  the  same  form 
and  at  right  angles  to  the  common  plane  of  the  basal  radii  (fig.  14,  Plate  XXIV.),  from 
Grantia  nivea,  Johnston. 
Spiculated  eectangulated  teieadiate. — This  form  of  defensive  spiculum  is  occasion- 
ally found  intermixed  with  the  equiangulated  triradiate  forms  in  several  species  of 
Grantia. 
Ensieoem  spiculated  equiangulated  teieadiate. — The  spicular  ray  is  at  right  angles 
to  the  common  plane  of  the  basal  radii,  but  not  of  the  same  form.  In  Grantia  ensata, 
Boweebank,  ms.,  a new  species  from  the  Island  of  Sark,  it  is  very  much  longer  and 
stouter  tlian  the  basal  radii,  and  its  diameter  is  considerably  increased  in  the  distal  third 
MDCCCLVIII.  2 s 
