An organism has a distinct body plan that limits its size and shape. A body plan encompasses symmetry, segmentation, and limb disposition. Almost all animals have bodies made of differentiated tissues, which in turn form organs and organ systems. Animal bodies have evolved to interact with their environments in ways that enhance survival and reproduction.
Body Plans
Animal body plans follow set patterns related to symmetry. They can be asymmetrical, radial or bilateral in form.
- Asymmetrical: having a disproportionate arrangement of parts; exhibiting no pattern, such as a sponge.
- Radial: a form of symmetry wherein identical parts are arranged in a circular fashion around a central axis. This plane is found mostly in aquatic animals, especially organisms that attach themselves to a base, such as a rock or a boat, and extract their food from the surrounding water as it flows around the organism, such as sea anemone.
- Bilateral: having an equal arrangement of parts (symmetry) about a vertical plane running from head to tail. This plane is illustrated in a goat.
In order to describe structures in the body of an animal, it is necessary to have a system for describing the position of parts of the body in relation to other parts.
Common directional terms that are used to describe the position of body parts in relation to other body parts:
- Dorsal – Nearer the back of the animal
- Ventral – Nearer the belly of the animal
- Cranial or anterior – Nearer to the skull of the animal
- Caudal or posterior – Nearer to the tail of the animal
- Proximal – Closer to the body
- Distal – Further from the body
- Medial – Nearer to the midline
- Lateral – Further from the midline
- Rostral – Towards the muzzle
- Palmar – The walking surface of the front paw
- Plantar – The walking surface of the hind paw
Limits on animals size and shape
Aquatic animals tend to have tubular shaped bodies (fusiform shape) that decrease drag, enabling them to swim at high speeds.
Terrestrial animals tend to have body shapes that are adapted to deal with gravity.
Exoskeletons are hard protective coverings or shells that also provide attachments for muscles.
Before shedding or molting the existing exoskeleton, an animal must first produce a new one.
The exoskeleton must increase thickness as the animal becomes larger, which limits body size.
The size of an animal with an endoskeleton is determined by the amount of skeletal system required to support the body and muscles it needs to move.
Key terms
- fusiform: shaped like a spindle; tapering at each end
- exoskeleton: a hard outer structure that provides both structures and protection to creatures such as insects, Crustacea, and Nematoda
- apodeme: an ingrowth of the arthropod exoskeleton, serving as an attachment site for muscles
- endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage
Limiting effects of diffusion on size and development
The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. Diffusion is effective over a specific distance, so it’s more efficient in small, single-celled microorganisms. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective at completing all of these tasks. The center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.
Diffusion becomes less efficient as the surface-to-volume ratio decreases, so diffusion is less effective in larger animals. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.
Animal bioenergetics
An animal’s body size, activity level, and environment impact the ways it uses and obtains energy.
- An animal is endothermic (warm-blooded) if it maintains a relatively constant body temperature by conserving heat with the help of insulation.
- An animal is ectothermic if it does not have insulation to conserve heat and must rely on its environment for body heat.
- Metabolic rate is the amount of energy expended by an animal over a specific time. The rate is measured in joules, calories or kilocalories (1000 calories). In endotherms, it is described as the basal metabolic rate (BMR), while in ectotherms as the standard metabolic rate (SMR).
- Smaller endothermic animals have a higher BMR than larger endothermic animals because they lose heat at a faster rate and require more energy to maintain a constant internal temperature.
- More active animals have higher BMRs or SMRs and require more energy to maintain their activity. The average daily rate of energy consumption is about 2-4 times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR.
- A long period of inactivity and decreased metabolism (torpor) that occurs in the winter months is hibernation; estivation is torpor that occurs in the summer months.
Energy requirements related to the environment
Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism which allows animals to survive adverse conditions. Torpor can be used by animals for long periods. For example, animals can enter a state of hibernation during the winter months which enables them to maintain reduced body temperature.
If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals estivate to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface-to-volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day when they consume more energy to maintain their body temperature.
Animal body planes and cavities
Vertebrates can be divided along different planes in order to reference the locations of defined cavities.
- A sagittal plane divides the body into right and left portions; a midsagittal plane divides the body exactly in the middle.
- A frontal or coronal plane separates the front from the back.
- A transverse or horizontal plane divides the animal into upper and lower portions; it is called an oblique plane if it is cut at an angle.
- The posterior (dorsal) cavity is a continuous cavity that includes the cranial cavity (brain) and the spinal cavity (spinal cord).
- The anterior (ventral) cavity includes the thoracic cavity and the abdominal cavity.
- The thoracic cavity is divided into the pleural cavity (lungs) and pericardial cavity (heart); the abdominopelvic cavity includes the abdominal cavity (digestive organs) and the pelvic cavity (reproductive organs).
