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BIO-012 - Metazoa! Cheat Sheet by

a cheat sheet for all da animals out there

WHAT ARE METAZOA?

Metazoa is literally just another word for 'Anima­lia', the kingdom of animals. All of them are multi-­cel­lular eukary­otes, and they share some common behavi­ours.
All metazo­ans...
 ­ ­> consume organic material (so they are heter­otr­ophic)
 ­ ­> breathe oxygen
 ­ ­> can move
 ­ ­> can reproduce sexually
 ­ ­> start off from a blas­tula

A Nice Diagram of the Metazoa

THE EUMETAZOA

How do you know that it's a Eumeta­zoan?
It's an animal, and it's not a sponge. Thank God.
What groups is Eumetazoa split into?
Checking the diagram about now would be a good idea.

Eumetazoa can be radially symmet­rical, at which point you also know that they are diploblastic.

If they're not radially symmet­rical, then they must be bilate­rally symmet­rical, and you know that they are trip­lob­las­tic.

THE PROTOS­TOMES

Prot­ost­omes (who develop a mouth from the blasto­pore) are actually subdivided again into either Ecdy­soz­oa, or Loph­otr­och­ozoa.
How do you know it's an Ecdyso­zoan?
Ecdy­soz­oans undergo ecdysis, which is also more commonly known as molt­ing. This is when they shed their skin in order to grow. They do not exhibit spiral cleavage.
How do you know it's a Lophot­roc­hoz­oan?
Loph­otr­och­ozo­ans (this clade is also known as
Spiralia) display a 'spiral' cleavage pattern back in early embryonic develo­pment.

THE DEUTER­OSTOMES

Deut­ero­sto­mes (all of which develop an anus from the blasto­pore) are nice, because I don't think we have to know exactly how the group sub-di­vides any further.

They include all other kinds of animals, such as starfi­sh(­E­chi­nod­erm­ata) and verteb­rat­es(­V­ert­ebr­ata).
All Deut­ero­sto­mes exhibit radial cleavage.
 

SPONGES ARE SILLY

Sponges are really very strange 'animals'.

They don't have any kind of symmetry. That is how you know it's a sponge, and that it also doesn't have a proper ectoderm or endoderm!
Conseq­uently, a sponge does not develop a gastrula, unlike every other kind of animal. For this reason, all other animals are grouped into Eume­taz­oa.
Optional Discla­imer: I looked into it and it turns out that sponge embryos are way more compli­cated than I thought. So I can't say that they're not dipobl­astic, because it does have two germ layers present, but they're not called 'ectoderm' and 'endod­erm'. If you want to know, they're called the 'pinna­cod­erm', the 'choan­oderm', and then a weird non-ce­llular layer of gunk in between those two called the 'mesenchyma'.

The take-away here is that sponges are weird and don't develop a gastrula at all.

WHAT ARE DIPLOB­LASTS?

A dipl­obl­ast is an organism that forms two distinct germ layers:
Endo­derm: This is the inside of the gastrula. It forms the digestive system.
Ectoderm: This is the outside of the gastrula. This forms the outside of the body, and the 'nervous tissue'.
Examples of diplob­lastic animals include the Cnid­aria (jelly­fish, coral, etc) and the Cten­oph­ora (rotifers, various other semi-o­bscure marine invert­ebr­ates).

WHAT ARE TRIPLO­BLASTS?

The trip­lob­lasts are IMO where all the cool animals are. They're all bila­terally symmet­ric­al, unlike the radially symmet­rical diplob­lasts mentioned above.
How are triplo­blasts divided into smaller groups?
Triplo­blasts can be either grouped into Deut­ero­sto­mes or Prot­ost­omes. This is based on the way that the blastopore is incorp­orated into the rest of the organism as it grows.
In case you don't remember, the blas­top­ore is the first 'pore' created during gastru­lation, the inside of which is called the endod­erm.
Usually this pore penetrates through the whole organism to create the 'tube' that makes up the digestive system.
What is a Protos­tome?
Protos­tomes are all the species which develop a mouth out of the blasto­pore.
What is a Deuter­ost­ome?
Deuter­ostomes are all the species which develop an anus out of the blasto­pore. This is the one that we Homo sapiens are a part of.
 

WHAT IS EMBRYONIC DEVELO­PMENT?

What?
Embryonic develo­pment is what happens when two sex cells come together to produce a creature.
This is only a very 'generic' overview of the whole process.
Slight variations in certain parts of this process help us to determine what the evolut­ionary tree for metazoans would look like.
How does it start?
At first, there's just a zygo­te. A zygote is a single diploid cell that has been fertilized and is now ready to start growing into a creature.
How does a zygote grow?
The zygote starts off by 'cleaving' itself into more cells through mito­sis. This doesn't make it much bigger just yet, since it's literally just splitting itself into more cells, and not really adding much mass.

Eventually, the zygote ends up as a ball of several cells, now known as a moru­la.
What is blastu­lat­ion?
The next thing that happens is called blas­tul­ati­on, which is just when a morula becomes a blastula. This involves creating a hollow space inside the morula known as a blastocoel.

At this point, the blastula is only made of a single layer of cells.
What happens next?
A blastula then becomes a gast­rula, which is when some cells move inwards to make a cavity, which will be either the mouth or anus of the growing lil creature.

At this point, some cells have started to differ­entiate and become either the ectod­erm­(o­uts­ide), the endod­erm­(i­nside), or the mesod­erm(in the middle). Not all types of animal have all three at this stage!!

After that, organs start to develop, and it becomes much more specific to the species.
       

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