A Crash Course in Animal Characteristics

In Bio II, professors love spending a lot of time making you memorize taxonomical terms.

… A lot of time. I’m not kidding. Probably 3/4 of our time (if not more) is spent on memorizing phyla, classes, and orders. (If you’re unacquainted with these terms, just know that they’re categories into which organisms get categorized. The categories get progressively smaller in this order: domain, kingdom, phylum, class, order, family, genus, species.) The worst part is that none of us are absolutely certain that the information in our books is up-to-date, as every time we use Google to find references, we seem to find things with very different names, usually with “Formerly [term we just learned]” written off to the side.

However, as unpleasant as taxonomy can be, it is unavoidable. In Bio I, we learned all about Archaea, Bacteria and Protista. Our first test of this semester was a dreadful monstrosity that covered all of Plantae and Fungi… and that was only the tip of the taxonomical iceberg.

Now we’ve gotten to animals, of which there are simply too many.

Still, there are certain characteristics that many of them share, or at least, many that distinguish them from each other. Which, you may ask? Well, there are several outlined in my lecture notes. Sit down, hold on, and rev up Wikipedia. I’m about to show them to you.

1. Symmetry

Animals generally have some sort of symmetry. You can readily observe this if you look, for example, at your cat or your dog. Unless you happen to have a three-legged furry friend (my grandparents introduced me to one once), everything your pet has on one side it has on the other. If you were to draw a line down your dog’s back, it would have the same features on one side that it had on the other. This is a kind of symmetry.

There are three kinds of symmetry that animals can exhibit: asymmetry, radial symmetry, and bilateral symmetry.

Asymmetry, in my opinion, shouldn’t be called a type of symmetry at all. The “a-” part on “asymmetry” performs the same function that it does on other biological terms such as “asexual (not sexual)” and “anaerobic (not aerobic)”: it means organisms of this type lack symmetry. Sponges (phylum Porifera), as far as I know, are the only animals that belong here.

Radial symmetry is a kind of symmetry in which, if you sliced an organism like a pizza, every piece would be the same. These include hydras, jellyfish and sea anemones.

Bilateral symmetry is the kind of symmetry you can see in your dog, cat, snake, or tarantula, or even in yourself. It’s what it sounds like; the sides of these organisms are reflections of each other. A lot of organisms belong here, from tapeworms (yeck) to tarantulas, leeches to leopards. (Yes, that’s alliteration. You got a problem with that?)

Symmetry is one of the easiest characteristics of an organism to identify, but it’s far from the only one. Another less visible characteristic is commonly associated with symmetry…

2. Diplo- and Triploblasty

Developmental biology, inconveniently enough, plays a lot into the description of organisms. This is a particular case in which the development and tissue organization of an organism is of utmost importance. Before we can really understand what these two terms mean, though, we’ll need a miniature lesson in basic animal development.

Animal development starts at the level of a zygote, a single cell that results from the initial fusion of the gametes. This will undergo a rapid division of cells called cleavage, and eventually will result in a hollow mass of cells called a blastula. The blastula will then undergo gastrulation, where a folded, layered structure known as the gastrula will form. The layers are where germ layers come from.

Lost yet? Here, have a few pictures from The Wiki:


This is the development of the morula (mass of cells coming from the zygote) into the blastula…


… and this is the development of the blastula into the gastrula.

Those layers I mentioned? The ones you can see quite well in the pretty little picture above? Those have everything to do with whether an organism is diploblastic or triploblastic. You see, a diploblastic organism will only have two germ layers, the ectoderm and the endoderm. A triploblastic organism will have three: the ectoderm, the mesoderm, and the endoderm.

Guess which kind you are? That’s right, you’re triploblastic. You’re also bilaterally symmetrical, right? Well, turns out, that’s a trend: bilaterally symmetrical organisms are triploblastic.

All right, enough of this nonsense. I won’t let us linger long; I’m getting tired of this stuff, and I’m a bio minor. On to another characteristic, one that professors harp on almost constantly:

3. Coelom or No Coelom?

The organization of the germ layers of an organism plays into whether it has another characteristic used to distinguish organisms: a coelom. A coelom (SEE-luhm) is a body cavity in an organism that is found between its gut and outer covering and is lined completely with mesoderm. Organisms can be sorted into three categories based on their body cavities: acoelomates, psuedocoelomates, and ceolomates.

Acoelomates (there’s that “a-” again) are animals that don’t even pretend to have body cavities. A good example of this is the type of flatworm called Planaria (phylum platyhelminthes, class tubellaria). These are dorsoventrally flattened, and therefore don’t have cavities at all. (I could’ve used other flatworms too but, you know… I don’t like tapeworms.)

Pseudocoelomates have body cavities, but they aren’t completely lined with mesoderm. This category includes roundworms (among which are the heartworms that they treat your pets for) and rotifers.

True coelomates are animals that have a proper coelom (that is, they have a body cavity completely lined with mesoderm). This includes a lot of “more developed” organisms including—you guessed it—humans.

“Okay, okay, body cavities and embryos. Yee boy. Can we get back to something I can see, now?” Why certainly, reader! No discussion of animals would be complete without this…

4. Growth

Organisms are said to exhibit one of two kinds of growth: determinate or indeterminate. Just as it sounds, “determinate” growth means growth that eventually stops, whereas “indeterminate” growth is growth that keeps on goin’.

This is easier to see in plants, but it applies to animals, too. Most fish and some other things have indeterminate growth patterns, meaning, as my professor once said, “They keep growing until something eats ’em.” A lot of other organisms have determinate growth patterns, meaning they grow to a certain size and stop.

If you haven’t yet figured out which kind of growth you have, you can ask my friend, who stopped growing at much too small a stature for his liking.

“Determinate” and “indeterminate” are also terms used when talking about animal development—”determinate” means development in which every cell has a destined role, whereas “indeterminate” means that any cell could become essentially anything. Indeterminate cleavage is a characteristic of deuterostomes, which include chordates (and, thus, ourselves).

All right, take a deep breath. You’ve just read through 1160 words on animal characteristics. Good job! Give yourself a pat on the back! You already have a longer attention span than half of my Biology class (no, really, my professor frequently asks, “We are falling asleep?”). Give yourself a break. Go read about plants or something. The next thing we have to tackle is even more unwieldy than this—we’re going to study some animal taxonomy.

Questions? Suggestions? Compliments? Complaints? Go on, put them in the pretty little comments box. You know you want to.


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