Re: Explain why scientists think that sponges are closely related to protist.

dear Kimbra, Hi there!!!

You have asked a very good question, but with a little mistake. Sponges 
belong to a new kingdom called Protoctista and not protista.
This has been introduced recently. Earlier we used to follow Whittaker's 
Five kingdom classification system.

Basics:
All prokaryotic organisms are placed in the kingdom Monera for instance, 
bacteria.  All eukaryotic organisms that are unicellular organisms during 
most of their lives are placed in kingdom protista.  The remaining 
eukaryotic multicellular organisms are divided according to how they 
aquire energy and nutrients.  The Kingdom Plantae include all 
photosynthetic autotrophs.  The Kingdom Fungi includes all absorptive 
heterotrophs and the Kingdom Animalia includes all ingestive heterotrophs.

A recent trend is to redefine the protista as all eukaryotic organisms 
that are unicellular or relatively simple multicellular organisms, with 
little or no specializations of cell functions, for instance sponges.  
This new group, which includes the algae, is called the kingdom 
Protoctista.

A more recent suggestion by Lynn Margulis of Boston University is to 
avoid any attempt at classifying multicellular organisms on the basis of 
nutrition since these are autotrophic animals and heterotrophic plants.  
She suggests, instead that a five kingdom system be constructed by first 
seperating out the Monera, based on their prokaryotic cells and then 
defining the Kingdom Plantae as all organisms that begin life as haploid 
spores, and the Kingdom Animalia as all organisms that begin life when a 
haploid sperm and haploid egg egg unite to from a zygote which, after 
several cell divisions, becomes a blastula.  All organisms not belonging 
to these four Kingdom are to be placed in the kingdom Protoctista.
But, these classifications are still under discussion as it has not been 
accepted by everyone, it is really difficult to differentiate between 
Protist and Protocist.

Now you know why scientists think that sponges are closely related to 
protoctista, isn't it? Have a nice time reading

References:
1. Schaum's 3000 solved problems in Biology (page 157),Mc Graw Hill 
publication.
2. Biological Science, Cambridge publication, 3rd edition

shashank Harithsa 
Mad Scientist
July 4th, 2001

Moderator's note:
The relationships are now even more complicated.  Based on refined biochemical
and genetics comparisons, there are at least seven kingdoms of life on 
Earth: The prokaryotes are divided into Eubacteria and Archaea; and the 
eukaryotes are divided into Animalia, Plantae, Fungi, Chromista, and Protoctista.  
Although most phylogeneticists agree that "protoctista" should be further divided 
into three or more actual kingdoms.