30  Contamination  of  Drinking  Water.  {^JSSyfJwo.111, 
with  thickenings  in  the  walls  adjoining  the  vegetative 
cells  or  spores. — [a]  Heterocysteae. 
Cells  in  each  filament  undifferentiated.    No  hetefocysts. — 
{b)  Homocystese. 
(a)  Heterocysteae. 
t  Filaments  irregularly  interwoven  and  contorted, 
enclosed  in  a  definite  gelatinous  mass. — Nostoc. 
ft  Filaments  free  or  but  slightly  united. 
<p  Heterocysts  and  spores  intercalary. 
*  Filaments  free  or  united  in  a  formless 
mass  — Anabaena. 
**  Filaments  densely  agglutinated  in  fas- 
cicles often  of  considerable  size. — 
A phanizomenon. 
<p(p  Heterocysts  and  terminal  spores  contigu- 
ous.— Cylin  dros pei'mu  m . 
(b)  Homocystese. 
f  Filaments  simple,  with  an  evident  sheath. — 
Lyngbya. 
ft  Filaments  simple,  sheath  wanting  or  very 
slight,  plants  possessing  a  characteristic 
movement. — Oscillatona. 
Another  great  group  of  plants  which  of  late  years  has  been 
shown  to  be  of  importance  in  the  consideration  of  the  biology  of 
drinking  water  is  that  known  as  the  Diatomaceae.  By  some  botan- 
ists these  forms  are  believed  to  be  closely  related  to  the  Desmids, 
an  order  of  the  grass-green  algae,  which  seems  to  resemble  them  in 
certain  points  of  morphology  and  reproduction.  On  the  other 
hand,  there  are  those  who  maintain  that  the  diatoms  are  a  special 
class,  being  much  older  than  the  desmids,  and  that  the  points  of  re- 
semblance are  only  analogous,  not  homologous,  it  therefore  being 
impossible  to  regard  them  as  proofs  of  genetic  relationship.  What- 
ever their  affinity  with  other  algae  may  be,  they  certainly  constitute 
the  largest  group  of  any  of  the  aquatic  plants,  there  having  been 
more  species  of  diatoms  described  than  all  of  the  red,  brown  and 
green  algae  taken  together.  It  is  probable  that  a  great  many  of 
these  species  are  not  good,  but  even  alter  making  full  allowance  for 
such  duplication  there  still  remains  an  enormous  number  of  separate 
and  distinct  forms. 
