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Besides their names, what do horses, and seahorses, have in common? Although they look vastly different, they are both animals. Generally, we define animals as multicellular organisms that are motile, meaning capable of independent motion, consume organic matter for energy, and are capable of sexual reproduction. Species have evolved their different traits with specific functions to maximize their fitness in their habitats like lungs and gills for example. A good rule of thumb is that form follows function, meaning that the function of a feature can be predicted by its structure and appearance. For example, different finch species in the Galapagos Islands have evolved different beaks for specific diets like thin beaks to consume nectar and thick beaks to crack open large seeds. Thus, we can predict the diets of a species by observing structures involved with feeding.
Certain features mark branching points on the evolutionary tree and this is true all the way up throughout the whole tree of animal life. If we look at the basic structure of the evolutionary tree of animals, we can see that it is split into many different phyla with each phylum defined by major shared characteristics. Let's walk through these briefly.
The first major phylum is Porifera, simply meaning pore bearing. Stationary as adults, Porifera include filter feeders such as sponges. Lack of symmetry and distinct tissue layers separates Porifera from other groups and gives them enormous regeneration capacity where a whole organism can regrow from a small group of cells.
The next phylum, Cnidaria, include sea anemones and jellyfish and this group is characterized by their tentacles with specialized cells in the tips which they use to sting and subdue prey. Cnidarians are radially symmetric since they can be divided into equal parts along a central axis and possess two tissue layers, endoderm on the inside and ectoderm on the outside. This enabled Cnidarians to develop a simple gut with a single opening.
The Platyhelminthes, literally meaning flatworms, is our next group. Flatworms exhibit bilateral symmetry which means they have mirror image halves along a single axis. Bilateral symmetry enabled cephalization which is a concentration of sensory organs near the anterior end or head where these animals will continuously interact with their surroundings. Cephalization in turn enabled directional movement such as moving towards a food source. Platyhelminthes also developed a third tissue layer, mesoderm, which is sandwiched between ectoderm and endoderm.
As the animals grow more complex, it was the segmented phylum, Annelids like the earthworm, that developed a mesoderm-lined body cavity called the coelom. This new feature enabled further tissue specialization and paved the way for organ development.
Complexity of this specialization is increased in mollusks, a group which includes body plans as diverse as octopuses, snails, and clams. This group has many specialized organs covered and kept in place with a mantle.
Arthropods are arguably the most diverse phylum and include insects, crustaceans, and arachnids. They were the first group to develop jointed appendages and so were named accordingly. They also have sensory antennae and their bodies are divided into three specialized segments, head, thorax, and abdomen, all covered with an exoskeleton.
Finally, the Chordate phyla includes our old friend the seed eater and all other birds as well as tunicates or sea squirts, fish, amphibians, reptiles, and mammals. Chordates get their name from a novel physical trait they developed, the nerve cord, seen clearly in this typical Chordate embryo. This develops into the brain and spinal cord in most adults. They also have a cartilaginous structure, the notochord, which becomes the spine or backbone.
Here, we have characterized the main animal phyla in an extremely simplified manner. But as you can see, even within a single phylum, there are numerous differences in form and function to be found. This means that we can observe the diverse structures and physiologies of different members of a single phylum and predict their functions and life histories.
In this lab, you will observe crayfish and cricket structures and predict their functions.
Kingdom Animalia is composed of a range of organisms united by a set of common characteristics. Barring a few exceptions, animals are multicellular eukaryotes that move, consume organic matter, and reproduce sexually. Although these attributes are shared, species within this kingdom are also extremely diverse. This diversity is due to adaptation of each species to a different niche. The niche of a species includes the area, function, and interrelationship of that species with other biotic and abiotic factors in its environment. Niche specialization through evolutionary adaptation allows species to survive and reproduce effectively in their environment and reduces competition among species within the same habitat1-2.
Diversity in Kingdom Animalia has led to the classification of 36 distinct phyla, based on their evolutionary lineage. Seven of these phyla will be discussed, including Porifera, Cnidaria, Platyhelminthes, Annelida, Mollusca, Arthropoda, and Chordata. The first, phylum Porifera, is the earliest, simplest, most ancestral phylum. It includes multicellular, asymmetrical filter feeding sponges that lack distinct tissue layers. Interestingly, sponges can regrow after being broken apart to the level of a single cell. Next, increasing in complexity, the phylum Cnidaria includes jellyfish and coral. Members of this phylum are radially symmetric and diploblastic (possessing two tissue layers). These tissue layers are called the endoderm, which makes up the inner layer, and the ectoderm, which forms the outer layer. Organisms in phylum Cnidaria contain a simple gut with a single opening through which food enters and waste is excreted. Both Porifera and Cnidaria include organisms that are largely sedentary as adults. However, the remaining phyla have evolved adaptations that allow movement and more sophisticated behavior.
Following Cnidaria, there are two phyla of worms, termed Platyhelminthes and Annelida. Worms from both of these phyla breathe by exchanging gases across their skin. As a result, most worms have a large surface area to volume ratio and live in moist or aquatic environments. The skin of these organisms must remain moist to facilitate effective diffusion of gases. Another common characteristic of worms is their bilateral symmetry. This means that an organism can be symmetrically divided in half along a plane. Bilateral symmetry is seen not only in worms, but also in every phyla from this evolutionary point forward. Such symmetry is important to allow directed movement and to concentrate sensory organs near the anterior tip, or head, where they interact with their surroundings. Worms and later phyla also contain an additional tissue layer called the mesoderm that lies between the ectoderm and endoderm. As a result, these organisms are considered “triploblastic.”
Of the two phyla of worms, Platyhelminthes is the more ancient and simple. These include flatworms and trematodes, which may be free-living or parasitic. Like Cnidarians, they possess a simple gut with a single opening. Phylum Annelida is more complex, including earthworms, leaches, and polychaeta. These organisms contain a body cavity, called the coelom, between the gut and body wall. They are also distinct due to the segmentation of their bodies. Division of an organism into segments was an important evolutionary step, allowing adaptation of each segment to specialized forms and functions. Major evolutionary advances in this phyla include the development of anterior segments with appendages specialized for grasping and processing food as well as posterior segments with appendages specialized for locomotion.
The next phylum, Mollusca, contains highly morphologically diverse organisms, including octopuses, snails, slugs, and clams. There are few defining features for this group. However, all mollusks possess a mantle, which acts as a protective cover for the respiratory organs, digestive tract, and reproductive structures. Next, the most diverse group of organisms is phylum Arthropoda. This phyla alone makes up approximately three quarters of all living and extinct species known today, including crustaceans, insects, and arachnids. All members of this phylum have a hard, outer coating made of chitin, called an exoskeleton. This exoskeleton is periodically shed for a new, larger one to take its place. Like Annelida, arthropods are segmented. However, instead of multiple repeating sections, arthropods are broken up into three distinct segments called the head, thorax, and abdomen. Most have specialized sensing structures attached to the head called antennae.
Lastly, phylum Chordata includes all animals with a backbone and a few without. Two defining features that unite this group are the presence of a nerve chord and a notochord. The nerve cord is a major part of the chordate nervous system and the notochord is a cartilaginous structure that runs between the nerve cord and the digestive tract. Phylum Chordata includes tunicates (which lose their nerve cord and notochord after metamorphosis from the larval to adult life stage), fish, amphibians, birds, reptiles, and mammals (including humans)3.
The vast diversity in body structure, habitats, and behavior observed across all 36 phyla within Kingdom Animalia is truly astounding. To understand these differences better, it is important to consider the connection between form and function. “Form” simply means the shape, size, and substance of a structure or organism while “function” stands for the way in which that structure is used. Accordingly, the function of a structure can be predicted based on the form observed, or vice versa. For example, a bird specialized to crack hard nuts would be predicted to have a sturdy, sharp beak while a bird that eats fruit may have a smaller, more rounded beak.
Importantly, the form of an organism is constrained by its physiological requirements. For instance, all animals must consume and digest organic matter and exchange gas with their environment. As a result, any adaptations to an animal’s form must continue to facilitate these processes efficiently. For example, in smaller animals, diffusion of gases across the skin is possible and often useful. However, as the size of an animal increases, its surface area to volume ratio decreases until diffusion across the skin can no longer provide sufficient gas exchange. Such larger animals have developed complex systems to deal with gas exchange, such as the lungs of a whale or the gills of a fish. As a result, the morphology of gas exchange systems depends on both the size of an organism and its environment.
Similarly, it generally follows with other types of adaptations that form follows function, constrained only by the physiological requirement of the organism. However, there are cases in which form does not follow function. Vestigial structures are those that served a purpose in an organism’s ancestor but are no longer useful in their current form. Such structures have little to no impact on the fitness of an individual, and are thus not evolutionarily selected for or against within the population. As a result, some structures persist with limited functional capacity. For example, the human appendix, a small pocket at the end of the large intestine, is an evolutionary remnant from our ancestors. While this structure has been hypothesized to store beneficial gut bacteria, removal of the appendix is possible with no deleterious effects.
Among adaptations seen in Kingdom Animalia, many have developed multiple times throughout evolutionary history. Consider the wings of a bat and those of a bird as an example. These structures are not a result of evolutionary relatedness, but have instead arisen from convergent evolution4. Convergent evolution occurs when two organisms independently evolve similar traits due to similar environmental pressures or niches. The wings of bats, birds, and insets have all developed independently and are said to be “convergent traits.” Clearly, such traits are quite beneficial for the niches in which they arise.