Animal Diversity
All animals belong to a biological kingdom called kingdom Animalia. This kingdom is then broken down into over 30 groups, or phyla (plural form of phylum). About 75% of all species on Earth are animals. Besides the phyla, animals can be divided into two types: vertebrates and invertebrates.
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Animal development shows a variety of patterns and trends, largely dictated by factors such as multicellularity, symmetry, and germ layer formation. Early in evolutionary history, simpler organisms displayed radial symmetry and were diploblastic, composed of two germ layers. As complexity increased, triploblastic animals emerged, possessing three germ layers and exhibiting various body plans. The development of a coelom, or body cavity, further diversified animals into coelomates, acoelomates, and pseudocoelomates. Cephalization, the concentration of sensory organs and nervous tissue in the head region, became prominent in many species. Digestive systems also evolved, ranging from simple gastrovascular cavities to more complex alimentary canals. Circulatory systems likewise diversified, with some organisms possessing open circulatory systems, where haemolymph bathes tissues directly, while others developed closed circulatory systems, confining blood within vessels for efficient nutrient and gas exchange.
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| The following diagram depicts the phylogeny (lines of evolution over time) of various phyla and patterns of body design. In the next few lessons we will be looking at some patterns of animal design |
Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function.
Unicellular organisms include bacteria, protists, and yeast.
Multicellular organisms are composed of more than one cell, with groups of cells differentiating to take on specialized functions.
In humans, cells differentiate early in development to become nerve cells, skin cells, muscle cells, blood cells, and other types of cells.
We have already studied the unicellular organisms when we did micro-organisms.
All of the animals we are going to discuss in this module are multicellular.
When referenced in the study of biology, symmetry refers to the balance, or agreement, in dimensions of an organism.
Symmetry can be found via the use of mathematics by dividing the organism into as many equal parts as possible (which is typically two or more equal parts) along an axis.
For example, a human being could be divided into left and right halves, and a marine jelly could be divided into almost infinitely identical parts.
Almost all animals are observed to have some type of symmetry, with few exceptions to this rule.
TYPES OF SYMMETRY
There are multiple types of symmetry observed in the animal kingdom.
Symmetry is a characteristic in the animal kingdom used to classify organisms as well as determine common ancestors for these groups;
since most living organisms share the type of symmetry they present with their most recent common ancestor.
The types of symmetry in this lesson we will elaborate on include: asymmetry, bilateral, and radial symmetry.
Cephalization is the evolutionary trend toward concentrating nervous tissue, the mouth, and sense organs toward the front end of an animal.
Fully cephalized organisms have a head and brain, while less cephalized animals display one or more regions of nervous tissue.
Cephalization is associated with bilateral symmetry and movement with the head facing forward.
Cephalization provides three benefits to an organism:
For starters, it promotes brain development.
The brain serves as a command and control centre for organizing and controlling sensory information.
Animals can evolve complex neural systems and higher intelligence over time.
The second benefit of cephalization is that sense organs can be concentrated in the front of the body.
This allows a forward-facing organism to scan its environment more efficiently, allowing it to find food and shelter while avoiding predators and other dangers.
As the organism moves forward, the front end of the animal senses stimuli first.
Third, cephalization moves the mouth closer to the sense organs and brain.
As a result, an animal can quickly analyze food sources.
Predators frequently use special sense organs near the oral cavity to gather information about prey when vision and hearing are insufficient.
Cats, for example, have vibrissae (whiskers) that detect prey in the dark and when it is too close to see.
Animals have evolved different types of digestive systems to aid in the digestion of the different foods they consume.
Gastrovascular cavity
The simplest example is that of a gastrovascular cavity which is found in organisms with only one opening for digestion - an incomplete digestive system.
Platyhelminthes (flatworms), Ctenophora (comb jellies) and Cnidaria (coral, jellyfish, and sea anemones) use this type of digestion.
A gastrovascular cavity is typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”.
Ingested material enters the mouth and passes through a hollow, tubular cavity.
Cells within the cavity secrete digestive enzymes that break down the food.
The food is absorbed by the lining of the cavity and waste is excreted by the same opening the food was ingested from.
Alimentary canal
The alimentary canal is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other therefore a through gut or complete digestive system.
Earthworms are an example of an animal with an alimentary canal.
Once the food is ingested through the mouth and digested, the nutrients are absorbed, and the waste is eliminated as faeces, through the anus.
Vertebrates have evolved more complex digestive systems to adapt to their dietary needs.
The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates.
No Circulatory System
The simplest animals, such as the sponges (Porifera) do not need a circulatory system because diffusion across their body surfaces allows for adequate exchange of water, nutrients, and waste, as well as dissolved gases.
Organisms that are more complex but still only have two layers of cells in their body plans, such as jellies (Cnidaria) also use diffusion through their epidermis and internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides. The exchange of fluids is assisted by the pulsing of the sea jelly's body.
Open Circulatory System
For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved.
Most arthropods have open circulatory systems.
In an open system, an elongated beating heart pushes the haemolymph through the body and muscle contractions help to move fluids.
Closed Circulatory System
Closed circulatory systems are a characteristic of vertebrates.
In this system, blood circulates entirely within a series of vessels, such as arteries, veins, and capillaries, maintaining separation from the interstitial body fluid.
This closed-loop system enables precise control over blood pressure, efficient nutrient delivery, and waste removal, making possible the transport of oxygen and vital substances to tissues while ensuring metabolic stability throughout the organism.
There are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy.
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| TYPES OF SKELETAL SYSTEMS: | ||
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Exoskeleton
An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism.
For example, the shells of crabs and insects are exoskeletons.
This skeleton type provides defense against predators, supports the body, and allows for movement through the contraction of attached muscles.
As with vertebrates, muscles must cross a joint inside the exoskeleton.
Shortening of the muscle changes the relationship between the two segments of the exoskeleton.
Arthropods such as crabs and lobsters have exoskeletons that consist of 30–50% chitin(/ˈkaɪtɪn/ KY-tin), a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells.
The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster.
Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows. This results in the growth of the organism being sporadic.
Endoskeleton
An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms.
Endoskeletons provide mechanical support for the body, protect internal organs, and allow for movement through the contraction of muscles attached to the skeleton.
The skeletons of humans and horses are examples of endoskeletons.
The human skeleton is an endoskeleton that consists of 206 bones in the adult.
It has five main functions:
providing support to the body,
storing minerals and lipids,
producing blood cells,
protecting internal organs,
and allowing for movement.
Phylogenetic tree of Chordata
A phylogenetic tree, also known as a phylogeny, is a diagram that depicts the lines of evolutionary descent of different species, organisms, or genes from a common ancestor.
Consider the following questions:
Are all Chordata vertebrates? Give a reason for your answer.
Which group of Chordata are extinct?
When did they become extinct?
During which Period in prehistory did the five groups of vertebrates we study, first evolve?
Of the five groups of vertebrates we study, which two are the most closely related?
Which of the five five groups of vertebrates we study have bony skeletons?
Reptiles, birds and mammals are tetrapods. What does this mean?
Search the internet for Agnatha and give an example.
Consider the following questions:
What extant (still alive today) group of reptiles is the closest relative to the birds?
Dinosaurs became extinct about 65 million years ago. Do you think birds had evolved before this time? Give a reason for your answer.
From the first phylogenetic tree above, what Era began 65 million years ago?
Homologous structures in animals are anatomical features that share a common evolutionary origin, indicating descent from a common ancestor. These structures may vary in form and function across different species but possess underlying similarities due to their shared ancestry.
A classic example is the pentadactyl limb. The pentadactyl limb, characterized by a structure consisting of five digits, is a homologous feature found in various vertebrates, including mammals, reptiles, amphibians, and birds.
Despite differences in function, such as grasping in primates, running in horses, swimming in whales, and flying in bats, the underlying skeletal pattern remains remarkably similar. This shared structure provides compelling evidence for common ancestry among these diverse groups, illustrating the evolutionary modification of a fundamental trait to suit various ecological roles.
Homologous structures provide compelling evidence for the theory of evolution by demonstrating the modification of ancestral traits to suit diverse ecological roles over time. They highlight the unity of life and the fundamental principles of descent with modification, reveling the evolutionary relationships among different species. | |
| The homologous structure of animal embryos is evident during early developmental stages, where shared features among embryos across different species reflect their common ancestry. For instance, the presence of gill slits and tails in the embryos of vertebrates, including fish, reptiles, birds, and mammals, illustrates their evolutionary heritage from a common aquatic ancestor. These homologous structures provide compelling evidence for the theory of evolution by showcasing the conservation of ancestral traits during embryonic development, despite eventual divergence into diverse adult form |
A classic example is the wings of birds and bats. Despite arising from different evolutionary paths, both birds and bats have evolved wings to achieve flight, demonstrating convergence in function to suit their aerial lifestyle. |
Similarly, the streamlined bodies of dolphins and sharks serve the same purpose of reducing drag in aquatic environments, yet they evolved independently. Analogous structures underscore the fascinating ways in which organisms adapt to similar challenges through different evolutionary pathways, highlighting the power of natural selection in shaping biological diversity.
The analogous design of extinct ichthyosaurs, dolphins, and sharks is evident in their streamlined bodies, which enable efficient movement through aquatic environments despite arising from separate evolutionary lineages
Hydrostatic Skeleton
A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment within the body, called the coelom.
The organs of the coelom are supported by the aqueous fluid, which also resists external compression.
This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism.
This type of skeletal system is found in soft-bodied animals such as Platyhelminthes, Cnidaria, and other invertebrates.
Movement in a hydrostatic skeleton is provided by muscles that surround the coelom.
The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement.
For example, earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism.
Shortening the muscles then draws the posterior portion of the body forward.
Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.
Exoskeleton
An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism.
For example, the shells of crabs and insects are exoskeletons.
This skeleton type provides defense against predators, supports the body, and allows for movement through the contraction of attached muscles.
As with vertebrates, muscles must cross a joint inside the exoskeleton.
Shortening of the muscle changes the relationship between the two segments of the exoskeleton.
Arthropods such as crabs and lobsters have exoskeletons that consist of 30–50% chitin(/ˈkaɪtɪn/ KY-tin), a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells.
The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster.
Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows. This results in the growth of the organism being sporadic.
Endoskeleton
An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms.
Endoskeletons provide mechanical support for the body, protect internal organs, and allow for movement through the contraction of muscles attached to the skeleton.
The skeletons of humans and horses are examples of endoskeletons.
The human skeleton is an endoskeleton that consists of 206 bones in the adult.
It has five main functions:
providing support to the body,
storing minerals and lipids,
producing blood cells,
protecting internal organs,
and allowing for movement.
The human skeleton is divided into two main parts:
The axial skeleton comprises the bones along the body's central axis, including the skull, vertebral column, and rib cage, providing support and protection for vital organs like the brain and heart.
In contrast, the appendicular skeleton consists of the bones of the limbs, shoulder girdle, and pelvis, facilitating movement and providing attachment points for muscles, tendons, and ligaments.
Together, these two skeletal divisions form the foundation for bodily support, protection, and locomotion.
IMMOVABLE (FIXED )JOINTS | ||||||||||
| These fibrous joints are found in the skull (cranial) bones. The joint is strong but doesn't allow movement. | |||||||||
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Slightly Movable Joints | ||||||||||
Cartilaginous joints allow slight, restricted movement. An example is the discs between the vertebrae of the spine. | ||||||||||
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Freely Movable Joints | ||||||||||
FREELY MOVABLE (SYNOVIAL) JOINTS | ||||||||||
| Freely movable joints are classified structurally as synovial joints and allow movement in various ways. Unlike fibrous and cartilaginous joints, synovial joints have a joint cavity (fluid-filled space) between connecting bones. This cavity is filled with synovial fluid. Synovial joints allow for greater mobility but are less stable than fibrous and cartilaginous joints. | |||||||||
| The synovial joint | |||||||||
| There are four main kinds of synovial (movable) joints: | |||||||||
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| The following table provides a summary of the different synovial joints with a model indicating the direction of joint movement: | |||||||||
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| The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body:
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| 1. PROTECTS INTERNAL ORGANS Bones also protect internal organs from injury by covering or surrounding them.
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| 2. PRODUCES BLOOD CELLS Red bone marrow is where the production of blood cells takes place. Red blood cells, white blood cells, and platelets are all produced in the red bone marrow. | |||||||||
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| 3. FACILITATES MOVEMENT
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| 4. STORES AND RELEASED MINERALS AND FAT | |||||||||
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| 5. SUPPORTS THE BODY | |||||||||
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