Sponges, corals, and pre-bilaterian life:
What is a phylum?
The classification below the metazoa kingdom. A phylum has a unique body plan. There
are around 33 phyla in Metazoa.
4 phyla come from early branches – Porifera, placozoa, Ctenophora, cnidaria. It is
unclear which of these evolved first.
Porifera (sponges): body plan, feeding, oddities:
Body plan:
Outer cells (pinacyte) – this is like the ectoderm. Inner layer of cells, like the
endoderm. Sponges are generally only two cells thick. There are pore cells
which span both layers, allowing water to travel through. They have
choanocytes (collar cells) scattered on the inner layer. Their flagella push
water away from themselves, causing water to come in through the pore
cells and out through opening. Collar cells are connected to each other
ensuring water remains flowing in the right direction.
Feeding:
There is an inner layer, but it is folded into many chambers, and water is then passed
out. Sponge feeding cells look a lot like choanoflagellates. They pump water through
and filter out any plankton for feeding on.
Oddities:
Deep-sea sponges – not enough food to use this same mechanism. They have a hook
like structure.
Placozoa: body plan, feeding, oddities:
Body plan:
Look like amoeba but are made up of 1000s of cells. Basically, just a blob.
Feeding: These cells are divided into two layers – there is an upper layer, some with cilia
and some without. The lower cell layer includes some secretory gland cells which
secrete enzymes. There are fibre cells between the two layers. They crawl over algae on
rocks in tropical seas.
Oddities: Probably about 20 species on DNA analysis, but some old textbooks will say
there is only 1 species.
,Cnidaria (jellyfish etc.): body plan, feeding, oddities:
Body plan:
Highly diverse, it includes jellyfish, sea anemones, box jellies,
coral, and hydroids. There are two life cycles – polyp and
medusa. Polyp = the cavity opening is at the top with a single
opening, both feeding and waste ejection is from here.
The medusa has much more mesoglea and is stiYer – the
medusa is the swimming stage. The stiY part allows the
contraction to push against something for movement.
Feeding:
Cnidarians are primarily predators. They
have nematocysts (stinging organelles) with stinging barbs.
Oddities:
In most (not all), there are two diYerent parts to life cycle: the
Medusa and Polyp. The Medusa has more contractile fibres and
motile whilst the Polyp stays still.
This does not determine which way up the organism is! E.g.
upside-down jellyfish which do not predate but have a symbiotic relationship with
photosynthetic organisms.
Some cnidarians live in colonies. The polyps bud and make a vast colony e.g.
Portuguese man o’war. Giant siphonophores! Corals are also colonies, but of the group
Anthozoa.
Another group is the myxozoans – worms! Parasitic cnidarian.
Ctenophora (comb jellies - briefly):
They also have radial symmetry with two cell layers. They are predators as well but use
sticky cells (colloblasts) rather than stinging cells. The glowing band are cilia, which
move like a Mexican wave.
,Summary:
Bilateria & bilaterian phylogeny:
DiEerence between phylogeny & taxonomy
Phylogeny means evolutionary tree basically. An evolutionary tree is a hypothesis as it is
not possible to go back in time. It is working out how diYerent organisms are related to
each other.
, A monophyletic group is called a clade – yellow
is the only colour on the image which matches
this. Monophyletic is a group which make up all
the descendants of a common ancestor.
Taxonomic groups should be monophyletic,
otherwise they need to be replaced.
Blue shows a paraphyletic group, this is sometimes useful – e.g. reptiles (technically
includes mammals), dinosaurs (includes birds)
Red = polyphyletic group e.g. things that fly, worms. Not very useful
Taxonomy = classification, giving names to things and diving things into sets. Many
people believe that taxonomy should use phylogeny.
You can only do with things that evolve (e.g. organisms, languages, viruses),
however you can do with everything.
How phylogenies are ‘reconstructed’
In the 1850s to 1980s, morphology was used to make phylogenetic trees. After this,
DNA sequencing has been used.
You can mix the two together in ‘total evidence’ however this has a lot of problems.
Characteristics used to compare morphology:
Symmetry:
Can have no symmetry, radial, or bilateral. Bilateral has a left and right, top and bottom,
and a front and back.
Germ layers:
Radial has 2 germ layers, whilst bilateral have 3 fundamental cell layers.
Body cavities:
Gut is not a body cavity – this is part of the outside
world. A cavity must be fluid based and part of the
structure. Left has no body cavity, whilst middle
and right have a body cavity. A coelom is a cavity
lined by epithelial cells.
Segmentation:
Repeating units – e.g. earthworms and centipedes.
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