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But first, a discussion on the origin of metazoa

Metazoans (= animals) - Criteria based on:

• Multicellular
• Motile at some stage of its lifecycle
• Heterotrophic
• Diploid organisms with development proceeding via a blastula

Theories origin of metazoans

Syncytial theory - Metazoa evolved from multinucleated ciliate based on:
(Syncytial refers to protoplasm that contains numerous nuclei not separated from each other by plasma membrane)

• Many protozoans are multinucleated, including ciliates

• Bilaterality in ciliates as in most metazoa
• Such an ancestor might be similar to ciliated 'planula' larva found in cnidarians (and some porifera and one ctenophore) and to acoel flatworms



Planula larva from a cnidarian

Problems with Syncytial Theory

• Radial symmetry occurs in "Radiates" (sponges, cnidarians, ctenophores, and placozoans)
• Little evidence of syncytial cells in most basal metazoans.

Colonial Theory -Metazoa evolved from a colonial protist

A colonial sphere of cells, termed a blastaea, invaginated to form cnidarian-like gut resulting in differentation of cell types.  
 

Support for Colonial Theory  

• Similar to the embryology of many metazoans
• Similarity of Cambrian fossil embryos to blastaea
• Similarity between Choanoflagellates and Choanocytes of sponges (not only basic morphology, but with respect to their internal organelles)
• Molecular evidence indicating a close relationship between chaonoflagellates and metazoans

Theory has been modified to explain planula larva and 'primitive' bilateral animals by ingression:

Numerous other theories have been proposed.   Recent  molecular data supports a monophyletic origin of metazoans. Does the evidence of monophylogeny suggest that two or more of these theories might be correct concerning the origin of metazoans?
 
 
 
 

Phylum Porifera: The Sponges
'Pore-bearing' animals.

Diverse: ~5,000 living species described.
 

Sponges have typically been considered an early branching event in the history of animals and may represent the ancestral body plan of all animals:
 

1) Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.

 

Simple cell and spicule  associations from  Doushantuo phosphorites  of southern China  (~570 million years ago)	Archaeocyaths are an extinct group  (~530 million years ago) and contributed  greatly to the creation of the first reefs.

 

2) molecular evidence: more closely related to other metazoans than any other living group, but other phyla are more closely related to one another than to poriferans. 
 
 
3) genetic/developmental evidence: Would you expect sponges to have Hox genes?  If they did, what would this suggest about whether the poriferan body is primitive?
 

4) The porifera may be a paraphyletic group with some taxa of sponges being more closely related to the other animal phyla than to other porifera taxa. 

What does this paraphyletic relationship suggests about the body plan of the ancestor of all animal phyla?

5) Body plan unique and simple among animals (see below)

Body Plan

Aristotle classified sponges as plants.

• unidirectional water flow through pores (ostia) in outer walls and out other larger opening(s) (oscula) driven both by the beating of flagella and passively.  How is water driven through sponges passively?

  What is being accomplished by this movement of water through the sponge?

A variety of growth form that can be loosely assigned into various body types:  

Asconoid

Syconoid

Leucanoid

 
 
• Body supported by a hard skeleton composed of spicules.

• Two layers of cells (though not considered true tissues due to lack of basal laminae, an extracellular matrix), with cells (acrheaocytes) in between not organized (gelatanous extracellular matrix mesohyl containing).  Sponges separated into cells can re-assemble themselves.  How does this differ from cells in most other metazoans?
  

• Lack of characteristics more typical of most other animals:
• No specialize physiological systems. How do they get by without them?
• No true musculature
• Often no symmetry

Natural History

• Mostly marine and sessile (intertidal to the deep-sea).  Sponges also provide a haven for a variety of parasites or commensals.

 


Environmental conditions can partially influence sponge morphology.
 
 
 

•   suspension feeders on very small (e.g. bacteria and suspended organic matter), some are symbiotic with alga, and one small group is considered carnivorous

• Asexual. buds or, in many freshwater species, gemmules (why are gemmules more common in freshwater species?)
  

 
• Sexual. Most are hermaphroditic
Sperm may be "broadcast"
Development usually internal.
Larvae free-swimming, and come in a variety of forms.


 

Metamorphosize when attaching to a substrate (external flagellated cells migrate inward to become choanocytes).

Class Hexactinellida - glass sponges

Siliceous spicules  (six intersecting rays) with secondary network forming trabecular net.   Live at very great depths and osculum is notably large.  Why?

Class Demospongia

Most diverse sponge group.

Spicules are of spongin (a collagen) and/or silicaeous, and includes 'bath sponges'

Variety of growth forms from encrusting sheets to branching stalks upright.
Includes the one freshwater exception, family the Spongillidae, the freshwater sponges (expel water by contractile vacuoles).  

             Commercial sponge harvest
 

Class Calcarea

Spicules of calcium carbonate and may be straight or composed of 3-4 rays.

Other classes:

• Some traditional classes have been separated into other sponge classes

• Others are represented in mostly found in the fossil record

Phylogenetic relationships within Porifera
Hexactinellids were widely viewed as an early branch within the Porifera, however there is controversy on the systematics of sponges based on recent molecular data.

 Ideas to consider from the sponges:

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