33.1 Animal Form and Function

Animal Body Plans and Characteristics

Animal body plans and structures

Animals display three main types of body symmetry, and the type of symmetry an animal has tells you a lot about its lifestyle and complexity.

Asymmetry is the simplest arrangement. Sponges have no definite shape or symmetry, and they lack true tissues or organs. They're essentially collections of specialized cells.

Radial symmetry means body parts are arranged around a central axis, like slices of a pie. Cnidarians (jellyfish, sea anemones, corals) are the classic example. The side with the mouth is the oral surface, and the opposite side is the aboral surface. This body plan works well for animals that are sessile (attached in place) or free-floating, since food and danger can approach from any direction.

Bilateral symmetry means the left and right sides of the body are mirror images. This is the most common body plan among animals, and it comes with several important features:

• Cephalization is the concentration of sensory organs and nervous tissue at the anterior (head) end. This allows for directed movement and enhanced sensory perception through structures like eyes and antennae.
• Directional terms to know: dorsal (back), ventral (belly), anterior (head end), posterior (tail end).
• Segmentation is the repetition of body units along the anterior-posterior axis. Annelids (earthworms, leeches) and arthropods (insects, crustaceans, arachnids) are segmented. Segmentation allows different body regions to specialize for different functions and increases flexibility of movement.

Body cavities (coeloms) are another key distinction among bilateral animals:

• Acoelomates (flatworms) have no body cavity; their organs are packed in solid tissue.
• Pseudocoelomates (roundworms) have a body cavity that is only partially lined by mesoderm-derived tissue, making it a "false" coelom.
• Coelomates (annelids, mollusks, arthropods, echinoderms, chordates) have a true coelom fully lined by mesoderm. This fluid-filled cavity provides space for organs to develop and move independently, aids in circulation, and cushions internal structures.

Factors in animal size and shape

Why can't animals just grow to any size they want? Several physical constraints shape what's possible.

Surface area-to-volume ratio is the big one. As an object gets larger, its volume increases faster than its surface area. Smaller animals have a higher ratio, which makes exchanging materials like oxygen, nutrients, and waste across their body surface very efficient. Larger animals have a lower ratio, so they need specialized internal systems (lungs, circulatory systems, kidneys) to meet their metabolic demands.

Diffusion limitations are directly tied to this. Oxygen and nutrients can only diffuse efficiently over very short distances (roughly a few cells thick). Once an animal exceeds a certain size, simple diffusion can't deliver materials fast enough, and waste builds up. That's why large animals require circulatory and respiratory systems.

Skeletal support maintains body shape and enables locomotion:

• Exoskeletons (arthropods) provide rigid external support but must be shed for the animal to grow, which limits maximum body size.
• Endoskeletons (vertebrates) provide internal support and can grow continuously with the animal.

Thermoregulation is also tied to surface area-to-volume ratio:

• Smaller animals lose heat rapidly because of their high ratio, so they must eat frequently to fuel their metabolism.
• Larger animals retain heat more easily because of their lower ratio, making it easier to maintain a stable body temperature.
• Insulation like fur, feathers, and fat layers helps reduce heat loss.

Locomotion requires body shapes matched to the environment:

• Aquatic animals (fish, dolphins) benefit from hydrodynamic streamlining to reduce drag.
• Flying animals (birds, bats) need aerodynamic body shapes and lightweight skeletons.

Energy requirements of animals

Metabolic rate is the amount of energy an organism uses per unit time. Two important measures:

• Basal metabolic rate (BMR) is the minimum energy needed to keep the body running at rest (for endotherms).
• Mass-specific metabolic rate is the metabolic rate per unit of body mass, which tells you how "intensely" each gram of tissue is working.

Allometric scaling describes how metabolic rate changes with body size. Metabolic rate scales with body mass raised to the 0.75 power:

$\text{Metabolic Rate} \propto \text{Mass}^{0.75}$

This means a mouse uses far more energy per gram of body mass than an elephant does. Larger animals are more energy-efficient on a per-gram basis, even though their total energy use is higher.

Activity levels directly affect energy demand. Foraging, escaping predators, and reproducing all increase energy requirements above the basal rate. Sedentary animals need less total energy than highly active ones.

Environmental factors also play a major role:

• Temperature and thermoregulation strategy:

  1. Ectotherms ("cold-blooded") rely on external heat sources. Their metabolic rate rises and falls with environmental temperature.
  2. Endotherms ("warm-blooded") generate internal heat to maintain a constant body temperature, which requires significantly more energy.

• Oxygen availability differs between environments. Water holds much less dissolved oxygen than air, so aquatic animals face greater challenges obtaining sufficient oxygen for metabolism.

• Food quality varies by diet. Herbivores consume food with lower energy density than carnivores, so they typically need to eat larger volumes. Omnivores eat both plants and animals, giving them access to a range of energy sources.

Physiological Systems and Homeostasis

All of the systems discussed above work together to maintain homeostasis, the stable internal environment that cells need to function properly. A few key processes contribute:

• Osmoregulation controls water balance and solute concentrations in body fluids. Freshwater animals, for example, constantly take in water by osmosis and must actively pump it out.
• Excretion removes metabolic waste products (like ammonia or urea) that would be toxic if allowed to accumulate.
• Circulation transports nutrients, gases, hormones, and waste products throughout the body, connecting all organ systems.
• Respiration handles gas exchange between the organism and its environment, bringing in oxygen for cellular metabolism and releasing carbon dioxide.

These systems become increasingly complex in larger animals, precisely because of the surface area-to-volume and diffusion constraints covered earlier. A flatworm can handle gas exchange across its skin; a mammal needs lungs, a heart, blood vessels, and kidneys all working in coordination.