The Lone Ranger
07-31-2008, 12:44 PM
An Introduction to Zoology
Chapter Two: Animal Characteristics:
What Is an Animal?:
As mentioned in the first chapter, “zoology” is the study of animals. So, the natural question to ask is, “What is an animal?” Well, animals are generally agreed to share four distinguishing characteristics. First, all animals are eukaryotes. Second, all animals are heterotrophs that ingest their food. Third, all animals are multicellular, and their cells are usually specialized to perform different functions. And finally, almost all animals show embryonic development.
We’ll consider each of these characteristics in turn.
Prokaryotic vs. Eukaryotic Cells:
http://www.freethought-forum.com/forum/gallery/files/5/0/prokaryote.jpg
A typical prokaryotic cell (in this case, from a bacterium).
As discussed in the previous chapter, bacteria and archaeans are prokaryotes. All other organisms are eukaryotes. The cells of prokaryotes are almost always much smaller than are those of eukaryotes, and they’re much simpler. As you can see in the illustration above, the genetic material of a prokaryotic cell is more or less scattered throughout the cell, and there are few specialized substructures (organelles) within the cell.
http://www.freethought-forum.com/forum/gallery/files/5/0/animalcell.jpg
A typical eukaryotic cell (in this case, from an animal).
Compared to a prokaryotic cell, a eukaryotic cell is almost always much larger. It is also far more complex. Various smaller structures called organelles perform specialized tasks within a eukaryotic cell. Typically, the largest of these organelles is the nucleus, where the cell’s DNA is found.
In some ways, eukaryotic cells are much more efficient than are prokaryotic cells. Their relative inefficiency may be one reason why almost all prokaryotes are single-celled (unicellular) organisms. In contrast, a great many eukaryotic organisms are multicellular, having bodies that are made up of many cells working together in a tightly-coordinated fashion.
[b]Nutrition:
Animals, like all living things (including plants), need food. Food provides two things; it provides the nutrients that are used to build body tissues and structures, and it provides the energy organisms require for growth, movement and maintenance.
Organisms that can absorb energy from their environments and use it to manufacture their own food are known as autotrophs (“auto” = “self” and “troph” = “feeding”). The best-known autotrophs are algae and green plants. These organisms use molecules such as chlorophyll to capture solar energy, which they then use to manufacture food. This process, of course, is photosynthesis (“photo” = “light” and “synthesis” = “to manufacture”).
Animals, of course, cannot manufacture their own food. Therefore, either directly or indirectly, animals must acquire their food from autotrophs such as plants. Organisms that cannot manufacture their own food and so must acquire it from other organisms are known as heterotrophs (“hetero” = “other”).
Fungi, too, are heterotrophs. Fungi typically feed by secreting digestive enzymes onto their food, then absorbing the digested material. Animals, by contrast, feed by ingesting their food. That is, animals take food into their bodies and digest it there.
http://www.freethought-forum.com/forum/gallery/files/5/0/ingestion.jpg
Animals must ingest their food in order to digest and assimilate it.
[b] Animal Cell Structure, Multicellularity, and Cell Specialization:
The cells of animals, unlike those of fungi or plants, are not surrounded by rigid cell walls. While this means that animals’ cells are not as durable as are those of plants or fungi, it also means that animals can be far more mobile than can plants or fungi.
All animals are multicellular, by definition, but what are the advantages of multicellularity? And why are there no unicellular organisms the size of dogs – or even of insects?
An important reason why no unicellular organisms are more than a few microns across in size has to do with the relationship between an organism’s surface area and its volume. When you double something’s size, if its shape remains constant its surface area increases by a factor of 4, but its volume increases by a factor of 8. This is true because surface area is a square function, while volume is a cubic function. So if you double something’s size, its surface area increases by 22 or 4 times. But its volume increases by 23 or 8 times when you double something’s size.
That surface area is a square function and volume is a cubic function is a critically important factor in determining how large a cell can grow. A cell’s surface area determines how quickly it can absorb the food and oxygen it needs in order to survive – and how quickly it can excrete poisonous metabolic waste products. Meanwhile, the cell’s volume determines how much food and oxygen the cell must absorb in order to survive – and how much poisonous metabolic waste it generates.
So you can easily see what happens as a cell grows larger. Since its volume increases far faster than does its surface area, a growing cell quickly reaches a point where, if it were to grow any larger, it would be unable to absorb food and oxygen fast-enough to keep itself alive – and/or it would be unable to excrete metabolic wastes fast-enough to keep from poisoning itself.
So, the only way that an organism can grow to more than a few microns across in size is if its body is made of many cells, each of which is small-enough that it can absorb food and oxygen (and excrete wastes) fast-enough to keep itself alive.
http://www.freethought-forum.com/forum/gallery/files/5/0/surface.jpg
As it grows larger, a cell’s volume increases
much faster than does its surface area.
http://www.freethought-forum.com/forum/gallery/files/5/0/volume_original.jpg
By dividing its body into many small cells, an organism can have a large
volume and a large surface area for absorption and excretion.
[break=Specialization of Cells]
So, all animals are multicellular. This allows them to grow to much larger sizes than can any single-celled organisms. Large size provides a number of advantages – not least of which is that, the larger you are, the fewer things there are that can eat you.
Another advantage of having a body that’s made of many different cells is that individual cells can become specialized to perform different tasks. Specialization of cells means that a multicellular organism can be much more efficient than can a unicellular organism of the same size, since each cell doesn’t have to perform all of the various tasks needed to keep the organism alive.
[break=Tissues and Organs]
In the great majority of animal species, cells that are specialized to perform the same task are organized into tissues. In turn, two or more tissues that work together to perform a particular task form an organ. There are four types of tissues found in animals: epithelium, connective tissue, nervous tissue, and muscle tissue.
Epithelium (or epithelial tissue) lines body surfaces, body cavities, and organs. Your skin, for instance, consists largely of epithelial tissue
http://www.freethought-forum.com/forum/gallery/files/5/0/epithelium.jpg
Some of the different types of epithelial tissue found in a typical animal.
[break=Connective Tissue]
Connective tissues bind body parts together, provide support and protection for the body, transport substances within the body, and fill body spaces. Connective tissue is easy to recognize because the cells of connective tissue are separated from each other by non-living material called matrix. The matrix may be a rigid solid, as it is in bone, for example. The matrix may have a gel-like consistency, as it does in loose connective tissue. The matrix can even be a liquid, as it is in blood.
http://www.freethought-forum.com/forum/gallery/files/5/0/connective-tissues.jpg
Some of the different types of connective tissues found in animals.
[break=Nervous Tissue]
Nervous tissue is unique to animals; no other organisms have anything like it. Nervous tissue allows the rapid transmission of information from one part of an animal’s body to another. It allows animals to sense changes in their environments, to quickly respond to environmental changes, to coordinate movements, and to store information – that is, to learn.
Nervous tissue consists of specialized cells called neurons that can rapidly transmit electrochemical impulses, plus various types of specialized supporting cells that are collectively called neuroglia. In most animals, neurons and neuroglia are bound together by connective tissue to form organs called nerves.
http://www.freethought-forum.com/forum/gallery/files/5/0/nervous-tissue_original.jpg
Nervous tissue. The large cells are neurons; some of the smaller cells are neuroglial cells.
[break=Muscle Tissue]
Muscle tissue, like nervous tissue, is unique to animals. Muscle tissue is specialized to contract when stimulated. (Usually, it contracts when stimulated by nervous tissue.) Muscle tissue, therefore, generates movement. Muscle tissue is typically bound together by connective tissue to form organs known as muscles.
Muscle tissue can be used to move substances within an animal’s body – for instance, the heart is made of muscle tissue, and its contractions move blood through the body. Muscle tissue can also be used to move the animal itself. In most cases, muscles that move an animal’s body work in conjunction with some sort of skeleton.
A skeleton is an internal or external structure that helps give shape to an animal’s body. In most cases, a skeleton also provides support against the pull of gravity and helps the animal resist external pressure. A skeleton can also be used to transmit or redirect the force generated by muscle contraction, and so greatly increases the efficiency with which an animal can move.
An internal, fluid-filled body cavity can function as a hydrostatic skeleton, because water cannot be compressed. A hydrostatic skeleton allows an animal to resist external pressure (and so, burrowing is possible), and can also be used to distribute muscle-contraction force.
A rigid skeleton not only provides support and protection, but if muscles are attached to it, it can function as a lever to redirect or magnify the force generated by contracting muscles. A rigid skeleton on the outside of an animal’s body is known as an exoskeleton. A rigid skeleton on the inside of an animal’s body is known as an endoskeleton.
http://www.freethought-forum.com/forum/gallery/files/5/0/muscle_original.jpg
Humans, like other mammals, have three types of muscle tissues.
Skeletal muscle (top left) moves parts of the body.
Cardiac muscle (top right) makes up the heart.
Smooth muscle (bottom) moves fluids through body tubes.
[break=Reproduction and Development]
[b] Reproduction and Development:
Some animals can reproduce asexually. Consider a hydra, for example. It is a small freshwater animal closely-related to jellyfishes. It can reproduce through “budding” – a smaller hydra, genetically identical to its parent, grows from the side of a hydra and eventually breaks free to live independently.
http://www.freethought-forum.com/forum/gallery/files/5/0/budding.jpg
A hydra reproducing asexually through budding.
Most animals, however, reproduce sexually. Sexual reproduction involves the production of two types of sex cells called gametes. Each gamete has only half the number of chromosomes that a normal body cell does, and is referred to as haploid.
Males produce small, mobile gametes called spermatozoa. (This is the biological definition of “male.”) Females, by contrast, produce larger, non-mobile gametes known as ova. (Ova are often, but incorrectly, referred to as “eggs.”) When a haploid spermatozoan fertilizes a haploid ovum, a zygote is formed. Since each gamete had half the normal number of chromosomes, a zygote has a full set of chromosomes and is diploid.
http://www.freethought-forum.com/forum/gallery/files/5/0/fertilization.jpg
Fertilization: note the size difference between the two types of gametes.
All sexually-reproducing animals start life as zygotes, and a zygote ultimately produces a multicellular animal. The process by which an organism goes from a unicellular zygote to a multicellular adult is known as development. The vast majority of animals undergo a distinctive type of development known as embryonic development.
[break=Embryonic Development]
While the details differ between different animal taxa, embryonic development occurs in five distinct stages. Those stages are gametogenesis, fertilization, cleavage, gastrulation, and organogenesis.
The first stage of embryonic development is gametogenesis, the formation of the gametes. This occurs in the gonads of the parents. The male gonads are the testes (or testicles), and the production of spermatozoa is known as spermatogenesis. The female gonads are the ovaries, and the production of ova is known as oogenesis.
One way or another, spermatozoa from the male are brought into contact with the ovum of the female. Fertilization is the result, the union of a haploid spermatozoan and a haploid ovum to form a diploid zygote.
Soon after fertilization, the zygote begins to divide. It first divides into two cells, then into four, then into eight, and so forth. These initial cellular divisions occur so quickly that the cells experience little or no growth between divisions. This rapid cell division without growth is referred to as cleavage.
Cleavage soon results in a solid ball of cells called a morula. As the cells continue to divide, the morula hollows out to form a ball of cells known as a blastula. The cavity the cells surround is known as the blastocoel.
One end of the blastula then begins to fold inward in the process known as gastrulation. As the blastula folds in on itself, the blastocoel is filled in, producing a structure that now has two layers of cells and an opening at one end. The blastula is now known as a gastrula.
The structure of the gastrula is critically important to the future development of the young animal. The pouch formed by gastrulation is known as the archenteron, and it will eventually form the animal’s gut. The opening in the archenteron that leads to the outside of the gastrula is known as the blastopore, and it will eventually form one of the openings of the animal’s gut – either the mouth or the anus.
The two layers of cells that make up the gastrula are the embryonic tissues. The inner layer of cells, the tissue that surrounds the archenteron, is known as endoderm (“endo” = “inside”). This layer of cells will ultimately form the tissues that line the animal’s digestive tract. The outer layer of cells is known as the ectoderm (“ecto” = “outside”); it will ultimately form outer tissues, such as the animal’s skin.
In the great majority of animal taxa, a third layer of cells forms between the endoderm and the ectoderm. This layer of cells is known as the mesoderm (“meso” = “middle”), and it will ultimately form many of the animal’s internal organs.
As development continues, the cells of the endoderm and the ectoderm (and the mesoderm, if present) begin to specialize, forming first tissues, and then organs. This is known as organogenesis, and once organs begin to form, the gastrula is referred to as an embryo.
http://www.freethought-forum.com/forum/gallery/files/5/0/development.jpg
Early stages of embryonic development in a typical animal.
http://www.freethought-forum.com/forum/gallery/files/5/0/starfish_development.jpg
Various stages of development in a sea star (“starfish”).
[break=Indirect Development]
In many (probably most) animals, the embryo develops into a larva. A larva is a sexually-immature animal that is distinctly different in form from its parents. Because they’re so different from their parents, larvae rarely eat the same foods; indeed, the larvae of a given species may live in entirely different habitats than do the adults. It’s widely believed that the dissimilarity of the larvae and the adults helps prevent them from competing for the same resources, and so allows for larger population sizes. This is advantageous because the larger is the size of a population, the less vulnerable it is to extinction. Eventually, the animal undergoes a process of metamorphosis in which its body form changes from that of a larva to that of a sexually-mature adult.
This sort of development – where the animal goes through a sexually-immature larval stage before metamorphosing into a distinctly different, sexually-mature adult – is known as indirect development. Butterflies and moths provide a good example of indirect development; think of the drastic differences between a caterpillar and an adult butterfly.
http://www.freethought-forum.com/forum/gallery/files/5/0/metamorphosis.jpg
In butterflies, a sexually-immature larva hatches from a fertilized egg.
The larva (caterpillar) eventually undergoes metamorphosis
to become a sexually-mature adult.
[break=Direct Development]
In animals with direct development, there is no distinctive larval stage. By the time they’re hatched or born, the young animals look more or less like smaller versions of their parents. Young, sexually-immature animals are sometimes called nymphs to distinguish them from sexually-mature adults, but there is no metamorphosis from one body form to another.
http://www.freethought-forum.com/forum/gallery/files/5/0/chickens_original.jpg
Birds (Phylum Chordata, Class Aves) undergo direct development.
[break=What’s Next]
Now that we have a good idea of what distinguishes animals from other living things, we’re ready to look at them in a little more detail. In the next chapter, we’ll discuss some of the relevant terminology. We’ll also take a look at the history of animal life.
Chapter Two: Animal Characteristics:
What Is an Animal?:
As mentioned in the first chapter, “zoology” is the study of animals. So, the natural question to ask is, “What is an animal?” Well, animals are generally agreed to share four distinguishing characteristics. First, all animals are eukaryotes. Second, all animals are heterotrophs that ingest their food. Third, all animals are multicellular, and their cells are usually specialized to perform different functions. And finally, almost all animals show embryonic development.
We’ll consider each of these characteristics in turn.
Prokaryotic vs. Eukaryotic Cells:
http://www.freethought-forum.com/forum/gallery/files/5/0/prokaryote.jpg
A typical prokaryotic cell (in this case, from a bacterium).
As discussed in the previous chapter, bacteria and archaeans are prokaryotes. All other organisms are eukaryotes. The cells of prokaryotes are almost always much smaller than are those of eukaryotes, and they’re much simpler. As you can see in the illustration above, the genetic material of a prokaryotic cell is more or less scattered throughout the cell, and there are few specialized substructures (organelles) within the cell.
http://www.freethought-forum.com/forum/gallery/files/5/0/animalcell.jpg
A typical eukaryotic cell (in this case, from an animal).
Compared to a prokaryotic cell, a eukaryotic cell is almost always much larger. It is also far more complex. Various smaller structures called organelles perform specialized tasks within a eukaryotic cell. Typically, the largest of these organelles is the nucleus, where the cell’s DNA is found.
In some ways, eukaryotic cells are much more efficient than are prokaryotic cells. Their relative inefficiency may be one reason why almost all prokaryotes are single-celled (unicellular) organisms. In contrast, a great many eukaryotic organisms are multicellular, having bodies that are made up of many cells working together in a tightly-coordinated fashion.
[b]Nutrition:
Animals, like all living things (including plants), need food. Food provides two things; it provides the nutrients that are used to build body tissues and structures, and it provides the energy organisms require for growth, movement and maintenance.
Organisms that can absorb energy from their environments and use it to manufacture their own food are known as autotrophs (“auto” = “self” and “troph” = “feeding”). The best-known autotrophs are algae and green plants. These organisms use molecules such as chlorophyll to capture solar energy, which they then use to manufacture food. This process, of course, is photosynthesis (“photo” = “light” and “synthesis” = “to manufacture”).
Animals, of course, cannot manufacture their own food. Therefore, either directly or indirectly, animals must acquire their food from autotrophs such as plants. Organisms that cannot manufacture their own food and so must acquire it from other organisms are known as heterotrophs (“hetero” = “other”).
Fungi, too, are heterotrophs. Fungi typically feed by secreting digestive enzymes onto their food, then absorbing the digested material. Animals, by contrast, feed by ingesting their food. That is, animals take food into their bodies and digest it there.
http://www.freethought-forum.com/forum/gallery/files/5/0/ingestion.jpg
Animals must ingest their food in order to digest and assimilate it.
[b] Animal Cell Structure, Multicellularity, and Cell Specialization:
The cells of animals, unlike those of fungi or plants, are not surrounded by rigid cell walls. While this means that animals’ cells are not as durable as are those of plants or fungi, it also means that animals can be far more mobile than can plants or fungi.
All animals are multicellular, by definition, but what are the advantages of multicellularity? And why are there no unicellular organisms the size of dogs – or even of insects?
An important reason why no unicellular organisms are more than a few microns across in size has to do with the relationship between an organism’s surface area and its volume. When you double something’s size, if its shape remains constant its surface area increases by a factor of 4, but its volume increases by a factor of 8. This is true because surface area is a square function, while volume is a cubic function. So if you double something’s size, its surface area increases by 22 or 4 times. But its volume increases by 23 or 8 times when you double something’s size.
That surface area is a square function and volume is a cubic function is a critically important factor in determining how large a cell can grow. A cell’s surface area determines how quickly it can absorb the food and oxygen it needs in order to survive – and how quickly it can excrete poisonous metabolic waste products. Meanwhile, the cell’s volume determines how much food and oxygen the cell must absorb in order to survive – and how much poisonous metabolic waste it generates.
So you can easily see what happens as a cell grows larger. Since its volume increases far faster than does its surface area, a growing cell quickly reaches a point where, if it were to grow any larger, it would be unable to absorb food and oxygen fast-enough to keep itself alive – and/or it would be unable to excrete metabolic wastes fast-enough to keep from poisoning itself.
So, the only way that an organism can grow to more than a few microns across in size is if its body is made of many cells, each of which is small-enough that it can absorb food and oxygen (and excrete wastes) fast-enough to keep itself alive.
http://www.freethought-forum.com/forum/gallery/files/5/0/surface.jpg
As it grows larger, a cell’s volume increases
much faster than does its surface area.
http://www.freethought-forum.com/forum/gallery/files/5/0/volume_original.jpg
By dividing its body into many small cells, an organism can have a large
volume and a large surface area for absorption and excretion.
[break=Specialization of Cells]
So, all animals are multicellular. This allows them to grow to much larger sizes than can any single-celled organisms. Large size provides a number of advantages – not least of which is that, the larger you are, the fewer things there are that can eat you.
Another advantage of having a body that’s made of many different cells is that individual cells can become specialized to perform different tasks. Specialization of cells means that a multicellular organism can be much more efficient than can a unicellular organism of the same size, since each cell doesn’t have to perform all of the various tasks needed to keep the organism alive.
[break=Tissues and Organs]
In the great majority of animal species, cells that are specialized to perform the same task are organized into tissues. In turn, two or more tissues that work together to perform a particular task form an organ. There are four types of tissues found in animals: epithelium, connective tissue, nervous tissue, and muscle tissue.
Epithelium (or epithelial tissue) lines body surfaces, body cavities, and organs. Your skin, for instance, consists largely of epithelial tissue
http://www.freethought-forum.com/forum/gallery/files/5/0/epithelium.jpg
Some of the different types of epithelial tissue found in a typical animal.
[break=Connective Tissue]
Connective tissues bind body parts together, provide support and protection for the body, transport substances within the body, and fill body spaces. Connective tissue is easy to recognize because the cells of connective tissue are separated from each other by non-living material called matrix. The matrix may be a rigid solid, as it is in bone, for example. The matrix may have a gel-like consistency, as it does in loose connective tissue. The matrix can even be a liquid, as it is in blood.
http://www.freethought-forum.com/forum/gallery/files/5/0/connective-tissues.jpg
Some of the different types of connective tissues found in animals.
[break=Nervous Tissue]
Nervous tissue is unique to animals; no other organisms have anything like it. Nervous tissue allows the rapid transmission of information from one part of an animal’s body to another. It allows animals to sense changes in their environments, to quickly respond to environmental changes, to coordinate movements, and to store information – that is, to learn.
Nervous tissue consists of specialized cells called neurons that can rapidly transmit electrochemical impulses, plus various types of specialized supporting cells that are collectively called neuroglia. In most animals, neurons and neuroglia are bound together by connective tissue to form organs called nerves.
http://www.freethought-forum.com/forum/gallery/files/5/0/nervous-tissue_original.jpg
Nervous tissue. The large cells are neurons; some of the smaller cells are neuroglial cells.
[break=Muscle Tissue]
Muscle tissue, like nervous tissue, is unique to animals. Muscle tissue is specialized to contract when stimulated. (Usually, it contracts when stimulated by nervous tissue.) Muscle tissue, therefore, generates movement. Muscle tissue is typically bound together by connective tissue to form organs known as muscles.
Muscle tissue can be used to move substances within an animal’s body – for instance, the heart is made of muscle tissue, and its contractions move blood through the body. Muscle tissue can also be used to move the animal itself. In most cases, muscles that move an animal’s body work in conjunction with some sort of skeleton.
A skeleton is an internal or external structure that helps give shape to an animal’s body. In most cases, a skeleton also provides support against the pull of gravity and helps the animal resist external pressure. A skeleton can also be used to transmit or redirect the force generated by muscle contraction, and so greatly increases the efficiency with which an animal can move.
An internal, fluid-filled body cavity can function as a hydrostatic skeleton, because water cannot be compressed. A hydrostatic skeleton allows an animal to resist external pressure (and so, burrowing is possible), and can also be used to distribute muscle-contraction force.
A rigid skeleton not only provides support and protection, but if muscles are attached to it, it can function as a lever to redirect or magnify the force generated by contracting muscles. A rigid skeleton on the outside of an animal’s body is known as an exoskeleton. A rigid skeleton on the inside of an animal’s body is known as an endoskeleton.
http://www.freethought-forum.com/forum/gallery/files/5/0/muscle_original.jpg
Humans, like other mammals, have three types of muscle tissues.
Skeletal muscle (top left) moves parts of the body.
Cardiac muscle (top right) makes up the heart.
Smooth muscle (bottom) moves fluids through body tubes.
[break=Reproduction and Development]
[b] Reproduction and Development:
Some animals can reproduce asexually. Consider a hydra, for example. It is a small freshwater animal closely-related to jellyfishes. It can reproduce through “budding” – a smaller hydra, genetically identical to its parent, grows from the side of a hydra and eventually breaks free to live independently.
http://www.freethought-forum.com/forum/gallery/files/5/0/budding.jpg
A hydra reproducing asexually through budding.
Most animals, however, reproduce sexually. Sexual reproduction involves the production of two types of sex cells called gametes. Each gamete has only half the number of chromosomes that a normal body cell does, and is referred to as haploid.
Males produce small, mobile gametes called spermatozoa. (This is the biological definition of “male.”) Females, by contrast, produce larger, non-mobile gametes known as ova. (Ova are often, but incorrectly, referred to as “eggs.”) When a haploid spermatozoan fertilizes a haploid ovum, a zygote is formed. Since each gamete had half the normal number of chromosomes, a zygote has a full set of chromosomes and is diploid.
http://www.freethought-forum.com/forum/gallery/files/5/0/fertilization.jpg
Fertilization: note the size difference between the two types of gametes.
All sexually-reproducing animals start life as zygotes, and a zygote ultimately produces a multicellular animal. The process by which an organism goes from a unicellular zygote to a multicellular adult is known as development. The vast majority of animals undergo a distinctive type of development known as embryonic development.
[break=Embryonic Development]
While the details differ between different animal taxa, embryonic development occurs in five distinct stages. Those stages are gametogenesis, fertilization, cleavage, gastrulation, and organogenesis.
The first stage of embryonic development is gametogenesis, the formation of the gametes. This occurs in the gonads of the parents. The male gonads are the testes (or testicles), and the production of spermatozoa is known as spermatogenesis. The female gonads are the ovaries, and the production of ova is known as oogenesis.
One way or another, spermatozoa from the male are brought into contact with the ovum of the female. Fertilization is the result, the union of a haploid spermatozoan and a haploid ovum to form a diploid zygote.
Soon after fertilization, the zygote begins to divide. It first divides into two cells, then into four, then into eight, and so forth. These initial cellular divisions occur so quickly that the cells experience little or no growth between divisions. This rapid cell division without growth is referred to as cleavage.
Cleavage soon results in a solid ball of cells called a morula. As the cells continue to divide, the morula hollows out to form a ball of cells known as a blastula. The cavity the cells surround is known as the blastocoel.
One end of the blastula then begins to fold inward in the process known as gastrulation. As the blastula folds in on itself, the blastocoel is filled in, producing a structure that now has two layers of cells and an opening at one end. The blastula is now known as a gastrula.
The structure of the gastrula is critically important to the future development of the young animal. The pouch formed by gastrulation is known as the archenteron, and it will eventually form the animal’s gut. The opening in the archenteron that leads to the outside of the gastrula is known as the blastopore, and it will eventually form one of the openings of the animal’s gut – either the mouth or the anus.
The two layers of cells that make up the gastrula are the embryonic tissues. The inner layer of cells, the tissue that surrounds the archenteron, is known as endoderm (“endo” = “inside”). This layer of cells will ultimately form the tissues that line the animal’s digestive tract. The outer layer of cells is known as the ectoderm (“ecto” = “outside”); it will ultimately form outer tissues, such as the animal’s skin.
In the great majority of animal taxa, a third layer of cells forms between the endoderm and the ectoderm. This layer of cells is known as the mesoderm (“meso” = “middle”), and it will ultimately form many of the animal’s internal organs.
As development continues, the cells of the endoderm and the ectoderm (and the mesoderm, if present) begin to specialize, forming first tissues, and then organs. This is known as organogenesis, and once organs begin to form, the gastrula is referred to as an embryo.
http://www.freethought-forum.com/forum/gallery/files/5/0/development.jpg
Early stages of embryonic development in a typical animal.
http://www.freethought-forum.com/forum/gallery/files/5/0/starfish_development.jpg
Various stages of development in a sea star (“starfish”).
[break=Indirect Development]
In many (probably most) animals, the embryo develops into a larva. A larva is a sexually-immature animal that is distinctly different in form from its parents. Because they’re so different from their parents, larvae rarely eat the same foods; indeed, the larvae of a given species may live in entirely different habitats than do the adults. It’s widely believed that the dissimilarity of the larvae and the adults helps prevent them from competing for the same resources, and so allows for larger population sizes. This is advantageous because the larger is the size of a population, the less vulnerable it is to extinction. Eventually, the animal undergoes a process of metamorphosis in which its body form changes from that of a larva to that of a sexually-mature adult.
This sort of development – where the animal goes through a sexually-immature larval stage before metamorphosing into a distinctly different, sexually-mature adult – is known as indirect development. Butterflies and moths provide a good example of indirect development; think of the drastic differences between a caterpillar and an adult butterfly.
http://www.freethought-forum.com/forum/gallery/files/5/0/metamorphosis.jpg
In butterflies, a sexually-immature larva hatches from a fertilized egg.
The larva (caterpillar) eventually undergoes metamorphosis
to become a sexually-mature adult.
[break=Direct Development]
In animals with direct development, there is no distinctive larval stage. By the time they’re hatched or born, the young animals look more or less like smaller versions of their parents. Young, sexually-immature animals are sometimes called nymphs to distinguish them from sexually-mature adults, but there is no metamorphosis from one body form to another.
http://www.freethought-forum.com/forum/gallery/files/5/0/chickens_original.jpg
Birds (Phylum Chordata, Class Aves) undergo direct development.
[break=What’s Next]
Now that we have a good idea of what distinguishes animals from other living things, we’re ready to look at them in a little more detail. In the next chapter, we’ll discuss some of the relevant terminology. We’ll also take a look at the history of animal life.