WEBVTT
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Welcome to the chapter
on sponge morphology.
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In this chapter we are going to learn
that while sponges have a variety of
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different body plans and different
forms and sizes, they share a
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number of morphological features that
allow us to recognize them as sponges.
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So what you see here are
three very different sponges.
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Clathrina lacunosa, Sycon ciliatum,
and Halisarca dujardini.
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They all share the same body axis, the
only body axis that they have, which is
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a basal-apical body axis, with the
base being where they are attached
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to the substrate and apical being the
part where their osculum is formed.
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The osculum, in plural oscula, is the
major opening - major exhalant opening
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- of the sponge body where
the water is being expelled
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away from the body, the water being pre-
viously drawn through the surface of the body.
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We're going to talk about the details
of this system in the next steps.
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What you see on the surface of many
sponges, such as this one called Sycon
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ciliatum or inside a variety of other sponges,
are skeletal elements called spicules.
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These are two very simple spicules built
of calcium carbonate that are characteristic
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for calcareous sponges, and there are
many other sponges that have much more
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complex spicules built of silica, which
is a very similar substance to glass.
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The simplest sponge body plan can, not very
surprisingly, be found in juvenile sponges.
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So this particular sponge is a juvenile of
Sycon ciliatum, it's approximately a millimeter
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or a couple of millimeters long. You can
recognise the spicules on the surface,
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and if you look at this cartoon representation
of this body plan, you will see that it's built of
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two epithelia, the inner one which is green
here and the outer one which is black.
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This very simple body plan is called
asconoid and this body plan is also
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characteristic for this sponge called
Clathrina lacunosa, where those simple
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tubes very similar to those basal tubes
that are present, their basic body plan
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that is present in the juvenile sponges are
found that are branching and anastomosing
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but they are still here meeting
in the single osculum.
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But as Sycon ciliatum and other sponges
from the genus Sycon are growing,
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they stop being that simple, they stop
being asconoid and they develop a
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different body plan called syconoid,
in which the same basic units of the
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outer epithelium and the inner epithelium
are folded repetitively around the main
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body cavity which is called the atrium.
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There is yet another body plan that is
recognized in sponges that is called
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a leuconoid body plan, found for example
in this sponge Halisarca dujardini
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where there is the inner digestive
epithelium that is forming little
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chambers inside and all those chambers
are connected by a system of canals
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that are again emptying to the atrium and
then the water is expelled through the osculum.
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So I told you there are epithelia, the
green epithelium which is the innermost
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epithelium and the black epithelium
which is covering the surfaces of the
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sponges, and what are those epithelia?
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The inner epithelium is called the
choanoderm, it is built of cells called
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choanocytes, we will learn more
about these cells in the next chapters.
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The outer epithelium is built of very
flat, very thin cells called pinacocytes,
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It's called the pinacoderm, and in between
those two layers you are finding the third
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layer that is not epithelial and it is often- there
are many different cell types present in them
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in this layer and this layer is called the
mesohyl, and if you look at all those
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different sponge body plans, you can
recognize that despite the different shapes
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the different levels of complexity of those
body plans, they are all built from those
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basic units: two epithelia, and the
mesohyl layer in between them.