The Kingdom Animalia (=Metazoa) is one of a handful of lineages rooted far back in the branching "tree" that represents the history of life on Earth. This lineage that is composed of those organisms we know as "animals" represents one of the three major origins of multicellularity (the other two large and diverse groups of multicellular organisms are the fungi and the green plants).
It is difficult to list characteristics that apply to all animals, since various branches of the animal tree have undergone a range of significant modifications. However, most animals obtain energy from other organisms. They generally feed on them as predators (killing and eating a prey item); parasites, including herbivores feeding on plants (feeding on their "prey" without killing it, at least not immediately); or detritivores (ingesting tiny bits of decomposing organic material such as fallen leaves). In contrast to animals, most plants make their own food, through the extraordinary process of photosynthesis, using energy captured from the sun; most fungi break down decaying organic material (without ingesting it) into its chemical constituents and absorb released nutrients. Animal cells lack a rigid cell wall (some form of which is typical of plants and most fungi) and their cell biology and physiology differ in a variety of ways from other organisms.
The diversity of animals is impressive. Zhang (2011; 2013) recently coordinated an effort to outline a classification scheme for all known animals and to estimate species richness (i.e., number of species) in different parts of the animal tree. Results from this publication are enlightening. More than 1.5 million animal species have been described (and many more continue to be discovered and formally described each year). The phylum Arthropoda (insects, spiders, crustaceans, etc.) accounts for around 80% of this total; around 2/3 of the total is accounted for by the insects alone. Well over a third of all known insects (and around a quarter of all known animal species!) are beetles: nearly 400,000 different species of beetles have already been described. Among the known species of insects are also nearly 120,000 Hymenoptera (ants, bees, and wasps) and nearly 160,000 Lepidoptera (moths and butterflies). More than 40,000 spider species and over 50,000 species of Acari (mites and ticks) have been described. Nearly 70,000 species of Crustacea (crabs, shrimps, barnacles, pillbugs, and many groups completely unfamiliar to those who don't study them!) are known. The Myriapoda (millipedes, centipedes, and relatives) includes around 12,000 described species. The Mollusca (clams, snails, octopuses, and relatives) is among the largest of the animal phyla, with nearly 120,000 known species. There are over 17,000 known species of Annelida (segmented worms, including earthworms, "polychaete" worms, leeches, and their relatives), Even some groups most people have never even heard of are quite diverse. For example, there are over 1000 described Acanthocephala, over 3000 Pseudoscorpiones, and more than 1500 Rotifera species (and rotifer specialists believe this last number may represent just a tenth or less of the true global rotifer species diversity). By comparison with these invertebrate clades, the generally more familiar vertebrate groups are less diverse, but many people may still be surprised to learn, for example, that there are around 32,000 species of described "fishes" and nearly 6,000 described mammal species. The numbers presented here are merely an appetizer. Anyone seriously interested in biodiversity will thoroughly enjoy studying the original volume by Zhang and colleagues which is freely available online.
Description of Animalia
Description of Animalia
Carnival of Animals
human wandering through the zoo
what do your cousins think of you
- archie the cockroach (aka Don Marquis)
A quality control inspector in a widget factory has an ideal form for widgets, and is only interested in how close each widget off the assembly line comes to this ideal. Too much variation means a widget won’t work, and something is wrong. Quality control inspectors are essentialists. For them to define what a widget is would be straightforward. The mathematical abstraction or ideal is the widget.
But in the world of living things there is no ideal form “animal” from which the “real” animals depart. Biologists have shifted from essentialism to “population” thinking. Evolution can only work if it has variations to work on. Ernest Mayr, the great Harvard evolutionary biologist, wrote: “For the populationist the type is an abstraction and only the variation is real.” (Mayr, 1959).
There is no lowest common denominator “animal”. What is real is all of the variation. All of the variations that make you unique from all of the other specimens of the species Homo sapiens, or all of the other members of the animal kingdom. The sum total of all of these differences makes up the definition of animal.
Obviously I can’t describe or list all of the ways every animal that exists, or has ever existed, varies from every other. Even identical twins have different fingerprints. And even if I could write such a definition of animal no one would be able to read it. It would be easier writing an article explaining what is a Blue Buck. This species of antelope (Hippotragus leucophaeus) became extinct around 1800, before any scientist could study it. There are only 4 specimens in museums. You can read the entire scientific literature in under an hour. And no one could dispute your findings because no one knows much of anything about this animal
In the children’s guessing game ‘animal, vegetable, mineral’ the world is divided into three groups. Science has revealed that there are more things in the world than are dreamt of in children’s games. The scientist Carl Woese divided life into three kingdoms - Archaea, Bacteria, and Eukaraya, with animals a subdivision of the later (together with protists, fungi, and plants). Other more recent taxonomic systems divide life into several kingdoms: bacteria, archaea, chromista, protozoa, animals, fungi, and plants (and sometimes add viruses as another kingdom for good measure). As of yet there is no consensus about how these kingdoms are related, or should be arranged, or even if they should all have the taxonomic status of ‘kingdom.’
Taxonomists divide the animal kingdom into between twenty and thirty phyla. One recent textbook, Kingdoms and Domains (Margulis, 2009) recognizes as many as 37 animal phyla. Some of the more visible or well known include the annelids (earthworms and leeches), arthropods (crustaceans, arachnids, and insects), chordates (including amphibians, birds, fish, mammals, and reptiles), cnidarians (corals and jellyfish), echinoderms (starfish and sea urchins), and mollusks (including snails, squids, octopods, and oysters). Some phyla, like poriferans (sponges), people sometimes don’t even realize are animals. For others that you may never have heard of see the gateway articles on invertebrates and worms.
The most diverse phylum is arthropods. Within this phylum the most diverse class is insects, and within this class the most diverse order is Coleoptera (or beetles).
Animals are likely monophyletic – that is to say they are all descended from a common ancestor (Conway Morris, S., 1998). (Mammal is a monophyletic group because it contains an ancestral species and its descendants. Although birds and mammals are both warm-blooded, ‘warm-blooded animals’ is not a monophyletic group because the last common ancestor of both was neither a bird nor a mammal, but a reptile.)
The cells in your body contain DNA, the hereditary information that allows them to reproduce copies of themselves. (The usual textbook analogy is that DNA is like a computer software program.) The genes that regulate limb development in humans and other mammals also regulate limb development in insects. The great diversity of animal life mentioned above is not so much the product of new genes. It is the product, via evolution, of ancestral genes being deployed in new ways during the development of an organism. In fact, all animals share in common certain genes called Hox genes, that plants and fungi lack. These might be thought of as traffic cop genes that specify positions, so that other genes know where to make an eye rather than an antenna (Erwin, 2002).
Animals, like plants and fungi, are eukaryotic – their cells contain a nucleus (which houses the DNA), and cell organelles like mitochondria that have special functions and structures.
Bacteria on the other hand are made of cells without a nucleus. As some bacteria can live inside of the cell walls of others, it is theorized that eukaryotes evolved from such symbiotic bacteria.
All animals are multicellular. Bacteria can form colonies, but each bacterium is more or less the same. In an animal’s body the cells are differentiated and have many forms and functions. Plants and fungi are likewise multicellular; in fact multicellularity probably evolved many times - in lineages separate from the three well known ones.
It was once thought that sponges may have evolved from different ancestors than the rest of the animal kingdom. They have pronounced differences from other animals, even though they are multicellular. What enabled mutlicellularity in animals to evolve? They evolved the biological equivalent of Velcro to stick cells together. These adhesive molecules and their receptors (the technical term is extracellular matrix or ECM for short) are something all animals have in common, even sponges, and is something that makes them a monophyletic group (Willmer, 1990).
If the fact that the cells that make up our bodies evolved from single celled ancestors seems academic, consider this. These single celled ancestors had to compete against one another, and try and multiply and produce more copies than other cells. But now they have evolved to be team players, and have to keep down their inherited ability to make more and more copies of themselves, so the whole body, with all its parts, will work as one.
But sometimes some of them break free of this rule, rather like when you were a kid playing dodge ball, there was always one other kid who hogged the ball and didn’t let anyone else have a turn. Well, when our cells break this bondage, although it is good for them to make lots of copies of themselves, it is not good for us. In fact we have a name for when this happens. We call it cancer.
If you think frogs get their energy by eating flies, rather than the other way around, this is not always true. The larvae of one fly species (Tabanus punctifer) bury themselves in mud, grab toads with hook-like mandibles, and eat them by sucking on the amphibian’s bodily fluids (Jackman, 1983).
The most visible distinction between the three multicellular kingdoms of animals, plants, and fungi is their source of energy. Just as your computer needs a power source to work, so living things need a source of energy. The sun is the ultimate source of energy for most living things, though there are bacteria that live deep underground, and communities of organisms around deep-sea hydrothermal vents, not dependent on the sun.
Plants produce energy by photosynthesis. They turn water, carbon dioxide, and sunlight into sugar. (Though not every plant photosynthesizes. Some, like Sarcodes sanguinea, don’t. They live as parasites of other plants.). Organisms like plants, or bacteria, which produce their own energy, are called autotrophic.
Fungi absorb the nutrients they need to survive from dead organic matter, by secreting enzymes. They are saprophytic.
Animals are called heterotrophic – they get their energy by ingesting organisms that produce it. Some are herbivores, they eat plants. Some are carnivores, they eat other animals that eat plants, and some are omnivores, they eat both. (But as is well known, there are some carnivorous plants, like Hellamphora heterodoxa, that eat animals.) The ecosystem thus contains a chain of matter-energy transfer known as the food chain or the food web. And not every animal is heterotrophic. There are some photosynthetic animals, like Elysia chlorita, that contain photosynthetic symbionts. They are solar powered animals (Rumpho, 2011). And there is even a species of sponge that metabolizes methane, Cladorhiza methanophila (Vacelet, 1995).
Animals, like plants, develop from an embryo. But animal embryos take the form a blastula – a hollow ball of cells that no species of plant, fungus, or anything else forms.
A sperm cell fertilizes an egg cell, and from this, the adult animal develops through a process of coordinated multiplication and differentiation. In some animals this process is internal, and they give birth to live young (most mammals, with a few exceptions like the platypus). Other animals, like birds, lay external eggs.
In some animal phyla (rotifers, nematodes, ribbon worms, flat worms), the cavity of this ball of cells or blastula goes on to form the body cavity of the animal.
In other phyla (called coelemates), the blastula folds inwards and forms a three-layered ball of cells with an opening.
These coelemates are further divided into two groups: the protostomes and the deuterostomes. In protostomes (like earthworms, mollusks, and arthropods), the first opening of the ball of cells becomes the mouth of the adult organism. In deuterstomes (like echinoderms and chordates, including humans) the first opening of the ball of cells becomes the anus. Later in development another opening is created and develops into the mouth.
Subsequent development varies greatly from species to species. Some insect species, for instance, undergo several stages of development, like the well-known butterfly metamorphosis from larvae to pupa to adult.
The development of many species is still unknown. Of all the species of sponges, the life cycles of only about a hundred have been studied. And a new discipline of evo-devo or the evolution of development has grown in the past couple of decades, to study how development has evolved.
The Canadian porcupine (Erithizon dorsatum) is nocturnal, so its behavior is not always apparent to us. Its mating season begins in the autumn. Although males exhibit sexual urges throughout the year, females exhibit a shorter period of sexual urges. They experience 3 or 4 months of gradually developing heat, during which time they will stimulate their genitals, by, for example, riding a stick. Males will sing from a “very inaudible low whine to a high-pitched piercing whine.” (Shandle, 1946). The height of the mating season occurs between November and January. Males will stand up on their hind legs. If a female is receptive to his advances, she will stand up as well. The male will then urinate over her. Often they repeat this behavior several times. The rest I will leave up to the reader’s imagination. Or readers can consult the scientific literature in the references.
In bacteria, reproduction, in the sense of making a copy of an organism, and in the sense of exchanging genetic information, are separated. A bacterium can make a copy of itself by splitting into two. No DNA has been exchanged with anything else. But bacteria can exchange DNA directly, like people sharing computer software. No new bacteria need be born in the process. The technical term is lateral gene transfer, or LGT for short. This led the Harvard scientist Richard Lewontin to describe the tree of life “…as an elaborate bit of macramé.” (Lewontin, 2001).
In animals, by comparison, making a copy is intertwined with exchanging genetic information. All animals are sexually reproducing, or have sexual ancestors. Some species, of aphids for instance, produce alternating sexual generations and asexual clones. In some species, hermaphrodites, (like the snail Felix aspersa), both sexes are contained within a single organism. Sometimes simultaneously, or sometimes sequentially. In other species, the sexes are separate organisms. Sometimes the sexual differences (or sexual dimorphism as it is called) in a species are not very pronounced; in other species they are considerable. In some insects for example, the strepsiptera, males have wings and fly and females are legless and wingless. In the spotted hyena (Crocuta crocuta) the female’s genitals mimic male genitals. In some species sex is genetically determined, and in others sex is determined by the environment. Even homosexual or bisexual behavior has been found in the animal kingdom (Bagemihl, 1999). (There is a vast literature about how sex evolved, and why it persists. For a recent survey see Ellgren, 2007.)
Looking at ourselves as members of the animal kingdom, the most undeniable fact about us is that we are mortal, though we spend a lot of time ignoring this or pretending otherwise. Why do animals die, and how did death evolve?
Like many creative artists, Woody Allen has been asked: “Would you like to survive through your work?” His reply: “No, I want to survive by not dying.” Instead of surviving through our children, or our work, why don’t we survive by just not dying?
Thinking of organisms as engineered machines can sometimes be misleading. For example, like all vertebrates, your digestive tract (how you eat), crosses with your respiratory tract (how you breathe) in your throat, putting you in danger of choking to death on honey garlic chicken wings. No engineer would design a machine in such a suboptimal way. But natural selection has to make do with historical legacies; it is less an ideal engineer than someone who tries to fix problems by using duct tape.
Organisms, unlike your car, can be self-repairing. You may ask, so why can’t your body just go on repairing itself and never wear out? The answer came from the scientist George Williams. Imagine you are car shopping. One model is very durable and doesn’t rust for ten years but that is because the material it is made of is heavier, more expensive, and makes it a gas-guzzler. Another is lighter, cheaper, and less gas greedy but will start to rust after only five years. Do a cost-benefit analysis. If the car gets stolen or totaled in an accident in five years then the extra cost of the non-rusting material will not pay off – it will have gone to waste.
Nature, likewise, has to make trade-offs. It chooses something more beneficial to a young organism than something that is good but spread out over a longer life span. This is because there is a greater statistical likelihood of being a young person or a new car, than there is being an old person or car. So adaptive traits that favor young organisms’ survival will, over time, get selected for. As George Williams wrote: “Senescence might be regarded as a group of adaptively unfavorable … changes that were brought in as side effects of otherwise favorable genes, and which have only been partly expunged by further selection.” (Williams, 1957). Hence the existence of what doctors call senescence and necrosis, or in plain English, old age and death. Nowadays bureaucrats and the medical profession have euphemisms for death like ‘Negative Patient Care Outcome.’ Only two things in life are certain, negative patient care outcome and taxes. (Again there is a large literature about how death evolved. For a recent survey of various theories see Mitteldorf, 2004.)
The most salient fact about the story of animal life is that for the longest time there wasn’t any. Animals are like people who arrive at a party fashionably late. The Earth is 4.6 billion years old. The oldest fossils are three and a half billion years old, and life perhaps first evolved around 3.8 billion years ago, though some evidence from biochemistry suggests it evolved earlier, perhaps even as early as 4.3 billion years ago. Between 4 to 3.8 billion years ago the Earth underwent a ‘late heavy bombardment’ (or LHB for short) of comets and/or asteroids, and if life did evolve earlier, it went through a bottleneck (Miller, 1995).
The earliest undisputable fossil evidence of animal life comes from towards the end of a period of time geologists call the Ediacaran, a mere 635-542 million years ago. The current record holder is a possible sponge-like animal from just before a worldwide glaciation called the Marinoan glaciation 650 Myr ago (Maloof, 2010). There is some evidence of trace fossils going back a billion years: fossils of tracks that may have been made by worm-like animals (Seilacher, 2007), but the evidence is debatable. (No sooner did I finish the final draft of this article, and then researchers at the University of Alberta announced the discovery of fossils of worm-like animals from about 585 Myr ago, making me behind the paleontological fashion curve. See Pecoits, 2012.)
In addition to fossil evidence, there is genetic evidence. By comparing the genes of various animals, it is possible to discover which are more closely related. It may also be possible to measure when the evolutionary divergence between them took place, using what has been dubbed the ‘molecular clock’ – if changes in the genetic code take place at a regular rate. Using this molecular clock method, it has been estimated that animals branched from all other living things between 700 Myr and one and a half billion years ago (Wray, 1996). But again, this evidence is still debatable. More recent studies have suggested the last common ancestor between a fruit fly and a mouse lived 600 – 540 Myr ago.
So the origin of animals, perhaps more than 700 Myr ago, and the subsequent diversification of them - during an event paleontologists call the Cambrian explosion - are separate events.
The paleontologist Dolf Seilacher has argued that the Ediacaran fauna were not precursors of modern animal phyla, but represents a separate experiment in animal evolution that produced many species unlike anything produced later (Xiao, 2009).
Since then there has been exponentially growing diversity, punctuated by several mass extinction events. The Cambrian explosion, 535-530 Myr ago, saw a vast radiation of animal diversity in a relatively short time (short geologically speaking of course, not in human terms). All of the 30 or so phyla mentioned above evolved by the end of the Cambrian. But there were far more phyla back then. Some paleontologists have identified as many as a hundred. So although the number of species has been increasing, the number of higher taxa has declined. Nor have new phyla evolved since.
( Gould, 1989)
The mother of all mass extinctions was the Permian one around 250 Myr ago that may have killed off as much as 90% of animal species. There were more than 50 species of mammal-like reptile at the time. If one of these species, thrynoxodon, does not eke its way through this extinction, mammals like us may not be here wondering just what an animal is. The more famous K-T mass extinction 65 Myr ago was almost certainly caused by the impact of a giant asteroid with the Earth, and if it had not occurred, dinosaurs might still be around, and mammals like us might be tree shrews that tyrannosaurs chow down on.
There is evidence that we are currently in the midst of another mass extinction, this one human caused. There is now a large literature on this subject. (See for example, Ward, 1995, or Wilson, 1999.)
Thus animals have been around for only a fraction of the time life has. And our species is only about 200,000 years old. As Mark Twain once quipped, if the Eiffel tower represented the history of life on Earth, we would be the lick of paint on the tip.
The cancer researcher Lewis Thomas suggested that:
“… introductory courses in science, at all levels…be radically revised.
Leave the fundamentals…aside for a while, and concentrate the attention
of all students on the things that are not known. You cannot possibly
teach quantum mechanics without mathematics…but you can describe
the strangeness of the world opened up by quantum theory. Let it be known
early on, that there are deep mysteries, and profound paradoxes…Teach…the
imponderable puzzles of cosmology. Let it be known, as clearly as possible,
by the youngest minds, that there are some things in the universe that lie
beyond comprehension, and make it plain how little is known...” (Thomas, 1995,
In that spirit, rather than describe or summarize more things we know about animals, here are some things we don’t know.
How many animals are there?
This may mean how many actual organisms there are, a measure called biomass. Or how many species are there? The answer to the first we don’t know. The Harvard entomologist E. O. Wilson, for instance, guessed that there might be ten quintillion insects in the world. The nematode roundworm is the most abundant animal on the planet.
How many species are there? So far taxonomists have described approximately one and a half million animal species (Zheng, 2011). To estimate the total number of species scientists extrapolate. For instance, they will put a tree in a giant baggie, fog it with insecticide, and then collect the dead insects. They then compare the insects already known to science, to those that are newly discovered, and try and extrapolate from this data, and guess the total number of species remaining to be discovered. Various such attempts have led to estimates of between 3 and 100 million species yet to be discovered. So science does not yet even know the order of magnitude, let alone the actual number of species.
These different estimates are due to different sampling procedures and statistical techniques. So it may be true that there are between 3 and 100 million animal species, but also not a very useful bit of information. It’s rather like a cake recipe that says ‘add between 3 and 100 cups of flour.’ In one recent study, Camilo Mora, of Dalhousie University in Halifax, has estimated that there are approximately 7.77 million animal species in the world today (Mora, 2011). And even if taxonomists have named a species there is still much to be learned about it. Nor should we forget that a statistically significant number of all of the species that have ever lived are extinct. In fact, probably 99% of animal species that have evolved are now extinct, and relatively few of those have left any trace in the fossil record.
bladder of Utricularia is predator of Animalia
Based on studies in:
Norway: Spitsbergen (Coastal)
This list may not be complete but is based on published studies.
- V. S. Summerhayes and C. S. Elton, Contributions to the ecology of Spitsbergen and Bear Island, J. Ecol. 11:214-286, from p. 232 (1923).
Evolution and Systematics
Defensins are naturally produced peptides that inhibit pathogen growth and degrade pathogen toxins by binding to the pathogens
“In addition to their bacterial membrane permeabilizing capacity, defensins have been shown to neutralize bacterial invasion by directly binding to bacterial toxins….Similar properties have been described for retrocyclins, a class of circular defensins found in non-human primates, which were shown to bind to the anthrax lethal factor with high affinity .” (de Leeuw and Lu 2007:69)
Learn more about this functional adaptation.
- Zou, G.; De Leeuw, E.; Li, C.; Pazgier, M.; Li, C.; Zeng, P.; Lu, W-Y.; Lubkowski, J.; Lu, W. 2007. Toward understanding the cationicity of defensins. The Journal of Biological Chemistry. 282(27): 19653-19665.
- de Leeuw, E.; Lu, W. 2007. Human defensins: turning defense into offense?. Infectious Disorders Drug Targets. 7(1): 67-70.
Mucins of animals stop invading pathogens by being coated with sugar chains that trap the invaders.
"Researchers at the University of Massachusetts and Yale University are looking for ways to trap viruses. In order to reproduce, viruses need to invade a host cell and replicate using the cell's own DNA-replication system. The researchers figured that if they could lure viruses to decoy cells, they could reduce the viral load enough for someone with HIV or other disease for that person's own immune system to successfully fight off the attack. Mucins are proteins found in most body fluids. They are coated with sugar chains that trap invading pathogens. Red blood cells also appear to act as pathogen traps. One approach is to coat nanoparticles with viral receptors. Another approach is to add decoy attachment sites to red blood cells. One advantage of using viral traps is it would be hard for viruses to evolve resistance to them." (Courtesy of the Biomimicry Guild)
Learn more about this functional adaptation.
The metabolism of animals oxidizes fat-soluble organic chemicals into excretable water-soluble substances, via P450 enzymes.
"The number of P450 genes cloned from various organisms such as animals, plants, yeasts, fungi, bacteria and sequenced is presently over 2000 and still increasing…P450s are major enzymes in drug metabolism in animal tissues and organs because they convert the pharmaceutics to more hydrophilic metabolites which are easily excreted into urine." (Hara 2000:103)
Learn more about this functional adaptation.
- Hara, Masayuki. 2000. Application of P450s for biosensing: combination of biotechnology and electrochemistry. Materials Science and Engineering: C. 12(1-2): 103-109.
The arterial walls of many animals resist stretch disproportionately by incorporating non-stretchy collagen fibers in a particular arrangement.
"In effect, Laplace's law rules out the use of ordinary elastic materials for arterial walls, requiring that an appropriate material fight back against stretch, not in direct proportion to how much it's stretched, but disproportionately as stretch increases. Which, again in obedience to the dictates of the real world, our arterial walls do--aneurysms, fortunately, remain rare and pathological. We accomplish the trick first, by incorporating fibers of a non-stretchy material, collagen, in those walls, and second, by arranging those fibers in a particular way. Thus, as the wall expands outward, more and more of these inextensible fibers are stretched out to their full lengths and add their resistance to stretch to that of the wall as a whole." (Vogel 2003:7-8)
Learn more about this functional adaptation.
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
Aquatic organisms move effectively through water by maximizing propulsion efficiency.
"It [the Froude propulsion efficiency] says that for highest efficiency, the velocity of the fluid issuing from the propulsive unit--paddle, propeller, jet, or whatever--should be as close as possible to the velocity of the craft...Clearly the way to maximize Froude propulsion efficiency consists of moving the largest possible mass-per-time (m/t) of fluid and giving it the least possible increase in speed (v2-v1). In practical terms that means maximizing S, the cross section of the propulsive flow stream."
While the Froud efficiencies "vary in quality and involve differenty underlying assumptions and simplifications, the picture that emerges is satisfyingly consistent with our expectations."
"* Moving water with undulating body, beating wing, or swinging tail beats squeezing water out of a jet, as anticipated. A squid may jet fast, but when it wants to go far, it's more likely to use its fins.
"* The same undulating devices do better than systems that move water back-wards with a paddling system, with its alternating power and recovery strokes. We'll return to this comparison between 'lift-based' and 'drag-based' propulsion in chapter 13.
"*Bigger (or at least moderate size) is better than smaller. Except for one questionable datum for a bacterial flagellum, no creature below about a centimeter in length does better than ηf = 0.5. The pernicious effects of low Reynolds number (chapter 11) cannot be denied.
"*The broad hydrozoan medusae (essentially small jellyfish) may use jet propulsion, but they do it by pushing out an especially large volume (relative to their own) through a wide aperture. So they have a much higher m and lower v2 than the other jetters, and thus evade most of the difficulty inherent in equations (7.5) and (7.6)." (Vogel 2003:142-143)
Learn more about this functional adaptation.
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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