Exploring Our Fluid Earth

Body Size and Complexity

In order to for life to exist, organisms must perform certain tasks, which includes but are not limited to acquiring energy, respiring, and removing wastes. The majority of this occurs by transferring materials across a cellular membrane. In a very broad sense, the larger an organism, the more resources can be obtained. However, there are constraints to how large an organism can grow. The volume of resources that are transferred across a cell membrane are related to the amount of surface area shown by an organism. As an organism increases in size, its volume also increases. In fact, for a spherical model organism, the surface area increases by the square of the radius while the volume increases by the cube of the radius. This presents a problem, because as an organism grows, its volume increases at a faster rate than its surface area. In order to increase body size, adaptations must evolve to increase surface area as well. Unicellular organisms rely primarily on diffusion of resources across their outer cell membrane, and many species have adaptations that increase their surface area to volume ratio. For example a diatom has a flat cell shape which increases relative surface area. However, there are not large unicellular organisms because they are still very constrained.

 

In contrast, while size is also limited in multicellular organisms, they have adaptations that allow them to grow large. In less complex multicellular organisms, which lack specialized tissues and organs, diffusion across the outer layer of cells is how resources are obtained. However, diffusion requires that the majority of cells be near the environment or the outside of the organism because as size increases so does volume, and diffusion alone is not enough to get resources to cells in the center of the organism. Certain jellyfish (phylum Cnidaria) and comb jellies (phylum Ctenophora) are able to grow large despite this constraint because their bodies are filled with a non-living liquid called mesoglea. Since the mesoglea is non-living, it does not require oxygen or other resources, so the organism can have cells concentrated on or near the outside of the organism and increase in size. Another solution to this problem is to increase surface area. Flatworms (phylum Platyhelminthes) for example are very thin so diffusion can occur over a large surface area but does not have to diffuse far within the organism. More complex multicellular organisms evolved over time to bring the resources closer to the cells in the body. This is done by adaptations like tissues and organ systems, which transport, oxygen, food, nutrients, and waste through the body. Body size and volume could increase because organisms were increasing surface area on the inside of the body.

 

Body Plans

 

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Fig. 3.7. Diversity of animal body plans

Image courtesy of multiple contributors from Wikimedia Commons

Organisms within the kingdom Animalia can be classified based on their body plan. An animal body plan is the basic structure of the organs and tissues within their bodies. In the animal kingdom there are two major themes within body plans: symmetry and the organization of tissues and body cavities. It is amazing that out of the approximately 1.73 million animal species living on Earth—95 percent of which are invertebrates—there exists a limited number of different body plans (Fig. 3.7). The diversity of organisms that has occurred within these constraints is a testament to evolution.

 

It’s important to understand body plans because they lay the foundation for the many adaptations that have evolved in animals. Once certain features have evolved, they also constrain any future adaptations. For example a building architect is told to design a house with four walls and a roof. There are an almost infinite number of houses that can be built, and they will differ in size, shape, color, and features, but they are all constrained by the basic blueprint of having four walls and a roof.

 

Body Symmetry

Most multicellular organisms have symmetrical body plans (Fig. 3.7). An axis of symmetry is an imaginary line that can be drawn through the center of a symmetrical object (Fig. 3.8). Structures on one side of an axis of symmetry mirror structures on the opposite side.

 

 

Fig. 3.8. Axis of symmetry for a trapezoid

Image courtesy of Oleg Alexandrov, Wikimedia Commons

Fig. 3.9. Asymmetrical body plans are rare in the animal kingdom, but they can be found in some sponge species such as red volcano sponge (Acarnus erithacus).

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

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Asymmetrical body plans are relatively rare in the animal kingdom. Some notable examples of body plan asymmetry can be found in sponges (phylum Porifera; Fig. 3.9). Most animals have either bilateral or radial symmetry.

 

Radial symmetry occurs when two or more axes of symmetry can be drawn through the center of the organism (Fig. 3.9). Radially symmetrical organisms are typically cylinder-shaped with body structures arranged around the center of the organism. Perfect radial symmetry is relatively rare but does occur in some sponges and cnidarians like anemones, corals and jellyfish (phylum Cnidaria; Fig. 3.10 A and Fig. 3.10 B). Sea stars, urchins, sea cucumber, and other animals in the phylum Echinodermata typically have five axes of symmetry (Fig. 3.10 B).

 

 

Fig. 3.10. (A) Lion’s mane jellyfish (Cyanea capillata; phylum Cnidaria)

Image courtesy of Arnstein Rønning, Wikimedia Commons

Fig. 3.10. (B) Individual polyps of a blueberry sea fan exhibit radial symmetry (Acalycigorgia sp.; phylum Cnidaria)

Image courtesy of Bernard Dupont, Flickr

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Fig. 3.10. (C) Moon jellyfish (Aurelia aurita; phylum Cnidaria)

Image courtesy of Hans Hillewaert, Wikimedia Commons

Fig. 3.10. (D) Tile sea star (Fromia monilis; phylum Echinodermata) exhibiting five-way or pentaradial symmetry

Image courtesy of Nick Hobgood, Wikimedia Commons

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Fig. 3.11. Oral or mouth side of a moon jellyfish (Aurelia aurita) with radial symmetry

Image courtesy of Alexander Vasenin, Wikimedia Commons

Radially symmetrical aquatic animals typically have an oral mouth surface and an aboral surface on the opposite side (Fig. 3.11). Sensory and feeding structures are often concentrated around the center point. From an evolutionary perspective, this would be advantageous because these organisms will be encountering stumuli and food from many directions.

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Bilateral symmetry occurs when an object has only one axis of symmetry (Fig. 3.12). Most animal phyla have bilaterial symmetry. Examples of bilaterally symmetrical animals include worms, insects, and molluscs. These organisms will typically have a front end known as the anterior and a back end known as the posterior. They also have left and right sides that mirror each other.

 

 

Fig. 3.12. (A) Lesser spider crab (Maja crispata)

Image courtesy of Daderot, Wikimedia Commons

Fig. 3.12. (B) Blue mussel (Mytilus edulis)

Image courtesy of Reiner Zenz, Wikimedia Commons

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Bilateral symmetry is typically associated with organisms that have locomotion or can move under their own power. Many bilaterally symmetrical animals have evolved feeding and sensory structures located at the front end of their bodies (Fig. 3.13 A and Fig. 3.13 B). Cephalization is the evolutionary development of an anterior head with concentrated feeding organs and sensory tissues in animals. Bilaterally symmetrical organisms typically move towards their environment at the anterior end. Cephalization likely evolved because it was advantageous to have feeding structures at the anterior end where food would be encountered as an organism moved forward. Similarly, it would be important to concentrate external sensory structures like eyes and antennae at the anterior end. It would be advantageous to have internal information processing centers, like the brain, closer to the anterior end to minimize the amount of time between the sensory stimuli and the brain’s response.

 

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Fig. 3.13. (A) Cephalization in a flatworm (phylum Platyhelminthes)

Image courtesy of Pei Yan, Flickr

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Fig. 3.13. (B) Cephalization in a Hawaiian bobtail squid (Euprymna scolopes; phylum Mollusca)

Image courtesy of Chris Frazee & Margaret McFall-Ngai, PLoS Biology Issue Image

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Fig. 3.13. (C) Cephalization in a peacock mantis shrimp (Odontodactylus scyllarus; phylum Arthropoda)

Image courtesy of Charlene McBride, Flickr

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Fig. 3.14. Bilateral symmetry in humans is approximate. The liver, stomach, colon, and several other organs are not bilaterally symmetrical in adult humans.

Image courtesy of Cancer Research UK, adapted from Wikimedia Commons

Symmetry is a relatively approximate measure. Not all organisms will show an exact mirror image match when comparing each side of an axis of symmetry. For example humans are considered bilaterally symmetrical because we have an axis of symmetry that bisects our body from our head to our feet (Fig. 3.14), but many of our organs, such as heart, kidneys, and stomach, are not perfectly symmetrically along that same axis. However, these are adaptations that have been built on a bilaterally symmetrical body plan.

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Tissue Layers and Body Cavities

 

Fig. 3.15. Gastrulation is the phase of embryonic development where three germ layers specialize and reorganize.

Image courtesy of Abigail Pyne, Wikimedia Commons

The presence of true tissue allows for complexity and increased body size within the animal kingdom. Tissue is an aggregation of similar cells that perform a specific function. For example, muscle tissue is made up of muscle cells that function to produce motion. Only a few animal phyla lack true tissue. Sponges (phylum Porifera) lack true tissue but are able to increase size through intricate branching and folding patterns. In animals that contain true tissue, the tissue layers in the adult are derived from embryonic tissue layers called germ layers. Germ layers are the tissues that occur after a fertilized egg has gone through several stages of cleavage, and cell aggregations are beginning to form tissue layers. This process in the embryo is called gastrulation (Fig. 3.15). During the gastrulation process, two germ layers develop: the ectoderm and the endoderm. The ectoderm is the germ layer that forms on the outside of the developing embryo (Fig. 3.16). The endoderm is the layer that develops on the inside of the embryo (Fig. 3.16).

 

The science of embryology, or developmental biology, examines how these germ layers develop into certain tissue types in the adult organism. Understanding how these germ layers are positioned in the embryo provides insight into how the adult organism will be constructed. The ectoderm tissue always develops into the outer skin layer and nervous system. The endoderm always develops into the lining of the adult digestive system. Diploblastic animals only have two germ layers: the inner endoderm and the outer ectoderm. Animals in the phyla Cnidaria and Ctenophora are diploblastic. The majority of invertebrates also have a third germ layer called the mesoderm (Fig. 3.15). The mesoderm is a layer between the endoderm and ectoderm that develops into skeletal structures, circulatory organs, and muscle tissue. Triploblastic animals have three germ layers and have a larger diversity of body plans compared with diploblastic organisms because of the additional mesoderm layer. The majority of them are bilaterally symmetrical.

 

 

Fig. 3.16. Cross-sectional diagram of endoderm, ectoderm, and mesoderm tissue germ layers in diploblasts and triploblasts

Image by Narrissa Spies

Triploblastic animals were able to become complex and diversify largely due to the presence of a fluid-filled cavity within their body. A body cavity is a “tube-within-a-tube” structure inside animal bodies (Fig. 3.16). The first tube is the outer tissue layer derived from the ectoderm. The second tube develops from the endoderm. In between the ectoderm and endoderm, there is a body cavity. The body cavity is also known as the digestive cavity.

 

While structurally simple, the body cavity has a variety of functions and allowed for development of new structures within the body plan. For example, organs such as gonads can be positioned within the cavity separate from the outer layer. Fluid inside the body cavity can also facilitate circulation of nutrients.

 

 

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Fig. 3.17. (A) Acoelom or lacking a fluid-filled body cavity (B) Coelom (C) Pseudocoelom

Image by Byron Inouye

Triploblastic animals are divided into three categories based on the type of body cavity they have. Acoelomates are triploblastic animals lacking a fluid-filled body cavity (Fig. 3.17 A). The flatworms (phylum Platyhelminthes) and ribbon worms (phylum Nemertea) are examples of acoelomates. Acoelomates have muscle tissue derived from the mesoderm germ layer filling the space between the endoderm digestive tract and outer ectoderm skin layer.

 

Coelomates are animals with a fluid-filled body cavity lined with tissue derived from the mesoderm germ layer. This lined body cavity is called a true coelom (Fig. 3.17 B). Coelomates are represented by many animal phyla including the Mollusca, Annelida, Arthropoda, Echinodermata, and Chordata. All vertebrates—including humans—are coelomates.

 

Pseudocoelomates are animals with a fluid-filled body cavity not completely lined with mesoderm tissue. The cavity is in between the mesoderm and the endoderm and is called a pseudocoelom (Fig. 3.17 B). The roundworms (phylum Nematoda) are examples of pseudocoelomates.