Back again with a
few more lower invertebrates, before we dive into the higher invertebrates and then
the vertebrates. We will look at the molluscs in some detail, as well as a
couple of less common phyla that were important in the past, such as the
brachiopods and bryozoans.
Brachiopoda is a group of
marine, filter-feeding, shelled organisms with much resemblance to bivalves
(mussels, clams, oysters, etc., which are molluscs and will be dealt with
later). They are bilaterally symmetrical triploblasts (see Part 4 if
that makes no sense). Brachiopods were hugely successful in the Palaeozoic era,
but suffered a rapid decline in the Mesozoic era, the time of the dinosaurs,
and have not recovered since. Today, there are only a few living species, the
most familiar being Lingula.
Lingula, an extant brachiopod. Image from http://www.aquarium.co.jp/shell/gallery/hyouzi.php?nakama=wansoku
Brachiopods can
be sessile – like sponges and sea anemones, attaching to the substrate with
their fleshy, tongue-like pedicle –
buried in the sediment – like many worms – or free-lying on the sea bottom
(though I think maybe only extinct brachiopods did that).
They take in
water into their shell in different ways, and filter out food particles with
their coiled, ribbon-like lophophore,
a special feeding organ, which they share with bryozoans and some other related
phyla. The lophophore is the unifying feature of a large group of phyla called
Lophophorata, which we will come across when discussing how all the animals we
have gone through are related. The lophophore is made up of what could be seen
as a main stalk and a myriad of tight, hair-like structures extending out at a
right angle, sort of like a brush, but flat. These hairs filter out food
particles from the water, and transport them to a cryptic mouth.
The lophophore (filter-feeding organ) of a brachiopod.
At first sight,
the brachiopods might seem indistinguishable from bivalve molluscs, especially
if you cannot see the insides (bivalves do not have a lophophore, but use
modified gills to filter food from the water). However, there is one simple way
of telling them apart by just looking at their shells (which is important in
fossils!). Brachiopods and bivalves are both bilaterally symmetrical, but the
symmetry planes are different: in
brachiopods, the symmetry plane goes across the shells, whereas bivalves have
the plane of symmetry between the shells. In effect, this means that the two
shell halves are identical in molluscs, but not necessarily in brachiopods – in
fact, brachiopod shell halves are always
different, the ventral (down-facing) being larger (but they sit upside
down, so the ventral appears as the top shell in life position).
Comparing the symmetry planes of brachiopods and
bivalve molluscs (‘valve’ is just a fancy word for an invertebrate shell).
Image from http://www.kgs.ku.edu/Publications/PIC/pic24.html
Bryozoa, moss animals, is another group that has seen its
glory days in the past, but now is quite uncommon. Like brachiopods, they are
marine, sessile filter-feeders (and of course bilaterally symmetrical
triploblasts). They also have a lophophore,
which they can extend out to the water to trap particles carried with the
currents, and retract to bring the food to the mouth. Their perhaps main
characteristic is a ‘crown’ of hollow tentacles.
Bryozoans. Image from http://goldfishgarage.blogspot.co.uk/2012/04/plumatella-bryozoan.html
Bryozoans are
typically colonial, like many corals
are, living together in tightly associated communities, helping each other out.
Bryozoan colonies are mostly clones
(maybe they should be said to be ‘clonial’?) stemmed from an original
individual. They may have specialised on carrying out particular functions in
the colony, such as taking in food, excreting waste, etc., so they work as a
super-organism.
Bryozoan colony. Image from http://www.starfish.ch/c-invertebrates/bryozoans.html
The common name
of bryozoans refers to them looking a bit like mosses, but being animals. So, I
guess that is a good clue: if you find something in the shallow seas that looks
like a moss, it is probably a bryozoan!
Now let us take
a close look at Mollusca, a very
diverse phylum of bilaterally symmetrical, triploblastic invertebrates. We know
many molluscs from our gardens (snails and slugs – gastropods) and dinner plate
(mussels, clams, oysters – bivalves – and squid and octopi – cephalopods).
Some, like the alien-looking monoplacophorans and polyplacophorans are probably
less familiar. Because these are generally very different-looking subgroups,
with rather different ecologies, we will look at them more closely than we have
for the subgroups of previous phyla.
But first: what makes molluscs molluscs? They all have a mantle, which usually forms a mantle cavity, where important organs and stuff happen, and a muscular ‘foot’ that is typically used for movement – or for staying put, in the case of bivalves. Another shared feature is the radula, a rasping tongue, full of chitinous teeth on one side, typically used to scrape various foods off a substrate; however, the radula has been lost (evolved away) in the bivalves, which have no use for it, being filter-feeders. Moreover, molluscs usually have shells, which are secreted by the mantle; cephalopods are a notable exception.
But first: what makes molluscs molluscs? They all have a mantle, which usually forms a mantle cavity, where important organs and stuff happen, and a muscular ‘foot’ that is typically used for movement – or for staying put, in the case of bivalves. Another shared feature is the radula, a rasping tongue, full of chitinous teeth on one side, typically used to scrape various foods off a substrate; however, the radula has been lost (evolved away) in the bivalves, which have no use for it, being filter-feeders. Moreover, molluscs usually have shells, which are secreted by the mantle; cephalopods are a notable exception.
Monoplacophorans
and polyplacophorans (chitons) are not very familiar, but not very abundant
either, so I will just show you some pictures for you to behold.
Drawing of a monoplacophoran mollusc. From the
underside (ventral view) just looks like a gluttonous, fat blob monster. Image
from http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/monoplacophora.php
Drawing of a polyplacophoran mollusc. I strongly
recommend you to google some images of live polyplacohporans: some are really
pretty, others rather frightening… Image from http://www.marlin.ac.uk/taxonomydescriptions.php
Gastropoda comprises our
familiar slugs and snails, the latter being the ones with shells (though
neither are formal groupings; shells have appeared and disappeared here and
there throughout gastropod evolution). They have a well-developed foot, used
for locomotion, sometimes aided by slime and/or cilia (hair-like structures).
Gastropod shells
are coiled, as opposed to the convex shells of mono- and polyplacophorans, and
the bilaterally symmetrical two-part shells of bivalves. This reflects a
process called torsion, which is
unique to gastropods. As they mature, the bulk of the internal organs,
including hearts, lungs/gills and intestines, is turned 180° inside the body.
This also causes nerve threads to cross over, so the brain needs to compensate.
In addition, this means that the anus points forward, and is located above the
head, in mature individuals. Torsion occurs in slugs as well, but is not as
conspicuous, as it is not mirrored by shell shape.
Drawing of a gastropod mollusc. Image from http://bio1151.nicerweb.com/Locked/media/ch33/gastropod-torsion.html
Bivalvia is probably the
most specialised mollusc group. As already mentioned, they have lost their
radula, otherwise so characteristic of molluscs, and their head is reduced to
virtually nothing, including the loss of eyes and other sensory organs. None of
that is needed, though, because they are filter-feeders. They use modified gills to filter food (instead
of oxygen) out of the water, and bring it to the mouth. Like brachiopods, they
keep their filtering organ inside the shell, and take in water through tubes
called siphons. Their foot is used
to either attach to a substrate, or bore into the sediment.
Drawing of a bivalve mollusc, with one shell half
removed.
Finally, we have
Cephalopoda, the squids and octopi.
They are the most advanced molluscs, some with brain capacities matching
mammals! Cephalopods are also notable for having evolved complex eyes
independently of vertebrates!
Cephalopods are
adapted for swimming rather than lumbering across a surface, making them the
fastest among the molluscs. The foot is modified into tentacles, used for
swimming, and the siphon can expel water explosively, giving a speed burst if
the animal needs to escape quickly.
Cephalopod mollusc. Image from http://bogleech.com/bio-ceph.html
The mouth,
containing a radula, has a sharp beak,
looking pretty much like that of a bird. Most cephalopods today have no shells,
but the now-extinct ammonoids and nautiloids (represented by a single living
genus: Nautilus) exhibited elaborate
shells in ancient times. A final epic thing about cephalopods is that they
basically have three hearts!!
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