Am)  PHYSIOLOGY  OP  THE  SPONGIAD^. 
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cerelellata^  Boweebank,  MS.,  I found  the  results  were  similar  to  those  obtained  from 
Halichmdria  ^anicea  and  mcrustans,  as  regards  the  spicula  of  the  skeleton ; but  in  the 
small  spinous  spicula  investing  the  gemmules  of  the  last-named  species  there  was  no 
apparent  alteration,  nor  could  any  indication  of  a central  cavity  be  seen. 
The  calcareous  spicula  of  Gmntia  convpressa  withstood  incineration  better  than  I 
expected.  The  surface  was  studded  with  numerous  little  vesicles  generated  by  the  heat, 
and  which  interfered  with  them  transparency;  but  they  retained  nearly  their  original 
colour  and  proportions ; and  it  may  therefore  be  concluded  that  they  contained  so  great 
a proportion  of  calcareous  matter  as  to  prevent  their  disintegration  by  heat. 
Many  of  the  forms  of  the  spicula  are  by  no  means  peculiar  to  the  Spongiadse ; but, 
on  the  contrary,  as  I shall  hereafter  show,  their  types  are  frequently  to  be  found  in  the 
more  highly  organized  classes  of  animals,  and  especially  among  the  Zoophyta,  the  Tuni- 
cata,  and  the  Nudibranchiate  Mollusca.  They  are  always  of  an  organic  type,  never 
crystalline  or  angular. 
Each  of  the  elongated  forms  of  spicula  may  be  said  to  be  composed  of  three  parts, 
the  base,  the  apex,  and  the  shaft  intervening  between  the  two ; and  generally  speaking, 
these  parts  may  be  readily  determined,  even  when  the  spicula  are  isolated. 
Each  species  of  sponge  has,  not  one  form  of  spiculum  only,  equally  dispersed  through- 
out its  whole  substance ; but,  on  the  contrary,  we  find  that  separate  parts  have  each  its 
appropriate  form ; and  thus  we  find  that  three,  four,  or  even  more  forms  often  occur  in 
the  same  individual ; and  in  Tethea  cranium  there  are  no  less  than  seven  distinct  shapes. 
But  these  differences  in  structure  must  not  lead  us  to  believe  that  every  strange  form  of 
spiculum  that  meets  the  eye  is  a normal  one : remarkable  variations  are  often  produced 
for  especial  purposes  in  the  construction  of  the  skeleton  or  for  other  objects ; and  in 
some  species,  Spongilla  lacustris^  for  example,  the  number  of  odd  malformations  that  are 
occasionally  found  is  very  remarkable.  The  size  also  of  the  normal  forms  of  spicula  will 
often  vary  to  a considerable  extent  in  the  same  sponge ; but  if  adult,  they  are  always  in 
accordance  with  the  type  form,  and  if  not  adult,  intermediate  states  of  growth  are  gene- 
rally present  to  assist  us  to  form  correct  conclusions  regarding  them.  The  forms  thus 
appropriated  to  the  different  parts  of  the  sponge  are  not  always  peculiar  to  certain  spe- 
cies, but,  on  the  contrary,  they  are  frequently  found  to  be  repeated  in  other  species 
differing  widely  in  their  construction. 
The  spicula  thus  appropriate  to  particular  parts  of  the  sponge  are  uniform  in  their 
general  characters  throughout  the  whole  of  the  Spongiadee,  and  a great  portion  of  them 
are  so  well  characterized  by  their  form,  as  to  enable  the  student,  when  once  well  acquainted 
with  their  peculiarities,  to  assign  each  readily  to  its  proper  place  in  the  sponge.  I 
purpose  therefore,  in  describing  them,  to  treat  of  these  organs  in  the  following  order : — 
1.  Spicula  of  the  skeleton. 
2.  Connecting  spicula. 
3.  Defensive  spicula. 
4.  Spicula  of  the  membranes. 
