Basic groups 6: Animals III

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. 

Image from http://invertpaleo.blogspot.co.uk/2011/07/brachiopods-versus-bivalves.html

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.

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.

 Image from http://kids.britannica.com/elementary/art-66016/Internal-structure-of-a-clam

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!!

...aaand another one

Okay, this time I'm making it a bit more complex, but hopefully it should still be fun and engaging!

Figure out which five key events in the evolutionary history of life you think are the most important. Write two paragraphs for each, the first explaining the event, the second analysing the results and why these are so important. Finally, pick three other key events that are not so important, and write down what they were and why they were not as significant as the first five. 

Next thing

Think about why some animal groups have remained virtually unchanged through their evolutionary history. Consider as many examples as you can come up with, and look for a common trend. 

Some examples to get you started:

Sharks
Turtles
Spiders
Scorpions
...

(and it's not "they all start with 's'" :p )

And another one...

Okay, this might be a tricky one if you are unsure, but it could be a ton of fun!

Pick your favourite dinosaur, find out what species it was likely to eat, and what species were likely to eat it, and make a food web for its ecosystem.

Another exercise

I still feel it is best to keep away from the computer as much as I can for a while, so we might keep letting you do the fun writing for some days. (Feel free to share if you want to! That would be super-cool!!)

Today, I thought you could find the closest fossil site to where you live, and do some research into the time and environment, as well as what sort of fossils you might expect to find there. Maybe you even want to go out and look for some! 

A hint is to look in brochures for sites of geotourism, or maybe websites for Sites of Special Scientific Interest. Google!

Exercise

Because my eyes have started to hurt when I'm on the computer too much lately, I thought this time better not write a long entry. Also, maybe a break from the group descriptions could be nice. So, this time, I will just give a question, and I encourage you to write down as much as you can think of. A mind map usually helps ideas flow, so if you are stuck, maybe try that out too!

What do you think the world would have been like today, if the dinosaurs had not been extinct 65 million years ago? 

Basic groups 5: Animals II

In this post, we will cover a two more worm phyla. In subsequent entries, we will see more miscellaneous ‘lower’ invertebrates, before moving on to the ‘higher’ invertebrates, and finally to the chordates.  

Annelida, the ringworms, is a group of advanced worms (but, worms being fairly primitive, they are still relatively simple animals). They are coelomate triploblasts with bilateral symmetry (if these terms are unfamiliar, please see Part 4). The coelom (body cavity) is filled with fluid, which acts as a skeleton: as muscles work against the incompressible fluid, the body changes shape, creating movement in relation to the environment. This type of skeleton is termed a hydrostatic skeleton, and is present in several animal phyla. The specific movements vary. The video below shows how a typical annelid (and earthworm) moves forward.

Locomotion of an earthworm. By compressing the sides of the coelom (using muscles circulating across the long axis), it expands in length; by relaxing the side pressure (and contraction of muscles running long the length of the worm), the worm shortens; the front anchors into the substrate between lengthening and shortening, which makes the annelid’s net movement forward.

This is indeed a very primitive way of moving, but I personally find it fascinating in its simplicity and apparent ingenuity.

The undulating contraction series that occurs during locomotion (peristaltic movement, in formal jargon) is made possible by the segmented body. The worm is divided into multiple segments, which contain repeated sets of certain organs and muscles. The circular muscles that create the contraction wave are repeated in each segment, and so are the muscles running along the long axis, which help pulling the animal together. The excretory organs, called nephridia (which are not much like our kidneys), are also repeated, in pairs. Extensions of the semi-centralised nervous system also spread out in each segment.  The blood flow of the circulatory system (internal transport of nutrients and gases, such as oxygen and carbon dioxide) is also organised with regard to the segments, while being connected throughout the animal.

Annelids also show a greater degree of cephalisation, compared to the more primitive platyhelminths (flatworms). In the front end, there is a concentration of nerve cells, a set of five ‘hearts’ (rings that function as pumps to make the blood flow through the vessels), a pharynx and an oesophagus, and specialised gut sections: a crop (for brief storage) and a gizzard (muscular section that can grind food material before passing it on to the intestines).

The leeches (Hirudinea) are a bit of an exception: their heads are simplified and modified into a blood-sucking device we are familiar with. They can use their suckers in front and back (the hind sucker is always larger) to move on a hard substrate, or swim around in water.

Unlike the poriferans, cnidarians and platyhelminths, the annelids are too complex to regenerate with such ease, although some are capable of recovering lost parts to some extent.

An annelid (member of the group Polychaeta), perhaps less familiar than the typical, 
bristle-less earthworm (Oligochaeta). Image from http://www.mediahex.com/Polychaete

A leech. Image from http://bio1151.nicerweb.com/Locked/media/ch33/leech.html

Nematoda is another group of worms, and, like the platyhelminths, they are mostly parasitic. They are pseudocoelomate, an intermediate between the acoelomate platyhelminths and the coelomate annelids (although they belong to different evolutionary groups). The nematodes are triploblastic, with a bilateral symmetry, but they are not segmented.

Perhaps surprisingly, the nematodes are actually more closely related to arthropods (insects, spiders, crustaceans, etc.) than to any worm phylum. (NB: once we have gone through the animal phyla, I will spend some time explaining how they are related, and hopefully it will create a clear picture of how the animals have evolved.) This is because they both have a hard external cuticle, a protective, multi-layered structure, composed primarily of collagen (a protein, common in connective tissues of many animals) in nematodes, and chitin (a sugar) in arthropods. The cuticle is smooth, and the nematodes have no obvious head, so there are basically no external features that characterise them – but, perhaps the lack of features itself is useful for recognising them!

A nematode. Image from http://www.flickr.com/photos/slider5/418135144/

The cuticle is rigid, and cannot grow together with the rest of the nematode, so it needs to moult – i.e. shed its cuticle and grow a new one that fits – just like arthropods; some snakes are also known to shed their skin.

This cuticle is layered in a way that makes it bendy, although inelastic, enabling the nematode to move. The nematodes only have muscles that run along the long axis, so they can only move by wriggling… *hrrmm* sorry, I should say waves of undulatory movement. A curious thing about nematodes is that they wriggle up to down, but swim on their side, so it looks like they wriggle sideways, like snakes.

Movement is possible thanks to the hydrostatic skeleton. This, coupled with the hard external cuticle, means that the nematode has high internal pressure. This has two important conesquences: first, the nematode requires a muscular pharynx in order to swallow food, because the intestines are under such pressure; second, if the cuticle breaks by accident, the nematode more or less explodes and dies. Therefore, the Nematoda does not possess any regenerative abilities, since damage basically leads to instant death.