Basic groups 4: Animals I

Now we finally got to the animals, the perhaps most interesting organisms to most of us. Animalia includes basically anything that is large and moves around, but also many organisms that are too tiny to sport with the unaided eye, and others that live attached to a substrate, usually the sea floor or a coral reef base.

In this post, we will briefly eyeball the more primitive phyla of kingdom Animalia. (If you are unfamiliar with the term phylum, and perhaps other taxonomic hierarchies, please see my old post about thetaxonomic system.) In subsequent posts, we will look at more advanced animals, leading up to the vertebrates.

I’ll start with the most primitive animal phylum: Porifera, the sponges. They are incredibly simplistic creatures, having no symmetry plan, and no true organised tissues or organs (no sensory organs, no heart, no lungs, no kidneys, no intestines, etc.). In essence, the sponges are clusters of cells, without much more organisation than having an internal empty space, or cavity; some form extending tubes.

Poriferans are sessile (i.e. living attached to a substrate; immobile) filter-feeders, so they don’t need any organs or specialised tissues to handle transport of food, waste or oxygen throughout the body (all of that occurs by diffusion – passive spread across a surface), and no sensory structures, since they filter food from the surrounding water – i.e. they are not predators that need to search for prey – and since they cannot move to escape potential dangers – i.e. there is no sense in making an effort to detect danger, if you can’t avoid it.

This might be a difficult lifestyle to imagine, and they probably strike you as incredibly boring and lame (or maybe you envy their relaxed and care-free existence?), but they can do something pretty awesome: because they are so simple, they can basically reassemble and/or regenerate (repair damaged parts) without hesitation or hindrance. What is even cooler is that if you totally disintegrate two different individual sponges into a mesh of loose cells, and mix them together, they will actually separate and reassemble as the distinct individuals they were/are!

Porifera. Image from http://rjfisherjoanides.pbworks.com/w/page/36528143/Porifera%20and%20Cnidarians%202

Cnidaria, jellyfish (Medusozoa) and sea anemones (Anthozoa, which includes corals), show a step up in complexity from the poriferans. They have true tissues, like all other higher animals; in Cnidaria, the tissues make up two layers: the external epidermis and internal gastrodermis, separated by a gelatinous goo called mesoglea. Typically, the epidermis faces outward and the gastrodermis folds inward and makes up a blind-ending gut cavity; food is filtered from water in the cavity, through the gastrodermis and into the mesoglea, where the chemical stuff happens.

While the cnidarians have a gut cavity, they do not have any internal organs: all internal transport occurs by diffusion across the mesoglea. However, they do have a simple nervous system: a network of unspecialised nerve cells, forming a so-called nerve net, which is probably useful for coordinating movement and internal signals (and, if I am not mistaken, they can detect basic environmental stimuli), although I doubt there is any central processing (i.e. thinking) going on.  

The cnidarian are radially symmetrical, meaning that if you them along any long axis, they will form two identical halves. In contrast to sponges, this means they have an up and down, but no left and right, like bilaterally symmetrical animals have (the remainder of the phyla we will consider here). This means that regenerating damaged parts requires certain sophistication, which the Cnidaria manages excellently.

However, the most obvious feature of the cnidarians might be their tentacles, which contain a unique type of stinging cell, called cnidocyte. The cnidocyte shoots out a spike, using water pressure, when stimulated. They are probably triggered by electrical signals from the nerve net. Like wasp stingers, the cnidocytes are single-use.

The cnidarian have two basic body forms: medusoid, which is typical for jellyfish, and polypoid, which is characteristic of sea anemones. However, species can shift between these forms throughout their life cycles, the mobile medusoid form being used primarily to spread out in the sea, and the cnidarian transforms into the sessile (immobile) mode when it settles.

Medusozoan cnidarians (jellyfish). Image from http://www.dramafever.com/news/yummy-jellyfish-turned-into-glow-in-the-dark-desert/

An anthozoan cnidarian (sea anemone). Image from http://hynpoikanikan.blogspot.co.uk/2011/06/sea-anemones.html

Radial versus bilateral symmetry. Image from http://ssrsbstaff.ednet.ns.ca/jcroft2/symmetry.htm

Platyhelminthes, the flatworms, are primarily disgusting parasites, a familiar example being the tapeworms, intestinal parasites that feed on our food until they are large enough to lay eggs and swirl out through our arse.

However, they represent the next step in basic animal evolution: they have three basic tissue layers, which is termed triploblastic, while the two-layered cnidarians are diploblastic. Platyhelminths have an external epidermis, an internal gastrodermis, and a mesodermis between these. In higher triploblastic animals, there is usually a space between the gastrodermis (or endodermis, as it is called in those) and the mesodermis, and this space forms the body cavity. Animals with a body cavity are termed coelomate, because the body cavity is named coelom; animals without a body cavity, i.e. the platyhelminths, are therefore acoelomate.

Moreover, the platyhelminths are bilaterally symmetrical, i.e. have a left and right side, as well as a front and back.

The mesoderm is the tissue layer that specialises into forming internal organs, muscles, and so forth, in triploblastic animals. The platyhelminths, however, only have a primitive excretory system (i.e. waste handling); there is no other internal transport system. Their gut is blind-ending, just like that of the cnidarians. In crude terms, this means they eat and defecate through the same hole.

The platyhelminths also show the beginning of head formation, or cephalisation, by the formation of a mouth and a concentration of nerve cells and sensory structures in the front.

There is one extreme exception to these general features: the cestodes, i.e. the tapeworms. These parasites are highly specialised, having lost their mouths and gut because they absorb nutrients from animal intestines through their skin. The only good their head does is hosting a sort of hook and sucker device, which they use to hold themselves attached to the intestine lining. 

The platyhelminths are capable of regenerating damaged tissue just like the cnidarians. 

A free-living (i.e. non-parasitic) platyhelminth. Image from http://kids.britannica.com/comptons/art-26318/Prostheceraeus-a-flatworm-of-the-class-Turbellaria

 

A cestode platyhelminth (tapeworm). These are too disgusting to show a real-life photograph of. Image from http://www.proprofs.com/flashcards/story.php?title=bilateriaflatworms

The next phylum will be the roundworms, Annelida, but let’s save the fun for tomorrow’s post! We will also go through a couple of more really basic phyla, before moving on to the more advanced invertebrates after that!