Animal Diversity

Main notes

Animals are the most diverse kingdom, with more than a million described species packed into roughly 35 phyla. This topic builds the body plan framework and the family tree of animal groups that later topics use as their stage. It connects back to evolution and classification and forward to the organ system physiology covered in the next two topics.

Body Plan Concepts

Animal body plans are described by a handful of features that recur in the same combinations across phyla. The first feature is symmetry. Radial symmetry means the body is arranged around a central axis, so any plane through that axis yields matching halves, as in a sea anemone. Bilateral symmetry means the body has a clear left side, right side, front, and back, as in a dog or a grasshopper. Bilateral animals usually show cephalization, the concentration of sense organs and a brain at the front end that meets the environment first.

Body plans also differ in germ layers, the embryonic tissue layers. Diploblastic animals, such as cnidarians, have two layers, the outer ectoderm and the inner endoderm. Triploblastic animals add a middle layer, the mesoderm, which forms muscles and most internal organs. Every animal except sponges and cnidarians is triploblastic. The mesoderm sits between the other two layers and gives rise to muscle, blood, bone, and most organs, so it is the layer that makes complex bodies possible.

A third feature is the body cavity. An acoelomate has no body cavity and packs its organs tightly against the body wall. A pseudocoelomate has a cavity lined by mesoderm on only one side. A coelomate has a true coelom, a cavity completely lined by mesoderm, which gives organs room to move and cushions them. Segmentation, the repetition of body units along the length, evolved independently in annelids, arthropods, and chordates.

Common Mistake: A pseudocoelom is not a small coelom. In a true coelom, mesoderm lines the cavity on both sides, while a pseudocoelom is lined on only one side. Roundworms are the classic pseudocoelomates, not small coelomates.

Finally, animals split by their early development. Protostomes form the mouth before the anus, and deuterostomes form the anus before the mouth. Deuterostomes include echinoderms and chordates, the group that contains humans. Most other animal phyla are protostomes. Body plans are not a ladder from primitive to advanced, and a sponge is not a failed sea anemone. Each phylum is adapted to its own way of life. The checklist below captures any body plan at a glance.

1. Check symmetry: radial or bilateral.
2. Count germ layers: two or three.
3. Assess the body cavity: none, pseudocoelom, or coelom.
4. Note segmentation and any appendages.
5. Classify development: protostome or deuterostome.

ELI-10

Picture a lunch box holding a sandwich bag. The air between the bag and the box is the body cavity, and the bag is the gut. Animals with this space can squeeze and move without crushing their organs. Animals without it are like a box packed with no room to spare.

ELI-10

Think of a snowflake that looks the same from every side. That is radial symmetry, like a sea anemone. A car has bilateral symmetry because its right half mirrors its left half. Animals with a head are usually the bilateral kind.

Invertebrate Phyla Overview

Zoologists recognize roughly 35 phyla of living animals, and about nine of them hold the vast majority of species. The invertebrates, animals without a backbone, fill all of these groups, and over 95 percent of described animal species are invertebrates. The table compares the major phyla.

Porifera are the sponges, sessile filter feeders whose cells act somewhat independently and that lack true tissues. Cnidaria includes jellyfish, corals, and sea anemones, all radially symmetric and armed with stinging cells called cnidocytes; many cnidarians alternate between a sessile polyp form and a floating medusa form. Platyhelminthes are the flatworms, acoelomate predators and parasites that include tapeworms. Nematoda are the roundworms, abundant pseudocoelomates found in soil, water, and hosts. Mollusca contains snails, clams, and octopuses, a coelomate group with a soft body and often a shell. Annelida are the segmented worms such as earthworms and leeches. Arthropoda is by far the largest phylum, with more than a million described species united by a chitinous exoskeleton and jointed appendages. Echinodermata includes sea stars and sea urchins, whose adults are radially symmetric but whose larvae are bilateral, and whose deuterostome development places them closer to the chordates than any other invertebrate phylum.

ELI-10

Imagine a block of apartments where every floor follows one floor plan. Each phylum is one floor plan, and every animal in the phylum builds its body from that plan. A crab and a beetle share the arthropod plan even though they look different. Learning one plan per phylum covers most of the animal kingdom.

Chordate Features

Chordates are the phylum that includes vertebrates, and every chordate shows four features at least some time in its life. The features are not always obvious, because in many groups they appear only in the embryo. The notochord is a stiff but flexible rod beneath the nerve cord that supports the body and gives muscles something to pull against. The dorsal hollow nerve cord runs along the back, and in vertebrates it develops into the brain and spinal cord. The pharyngeal slits are openings in the throat region that filter food in lancelets, become gills in fishes, and form jaw and ear structures in land vertebrates. The post-anal tail extends beyond the anus and provides propulsion in swimming forms.

The two invertebrate chordate groups are the lancelets and the tunicates. Lancelets keep all four chordate features as adults. Tunicates show the features as larvae and lose most of them when they settle onto the sea floor. Vertebrates are chordates that replace the notochord with a backbone during development. In adult humans the notochord survives only inside the disks between the vertebrae, and the pharyngeal slits are remodeled into the jaws, the middle ear, and the throat.

Common Mistake: Chordate and vertebrate are not synonyms. Tunicates and lancelets are chordates with no backbone. Only vertebrates replace the notochord with a vertebral column.

ELI-10

Imagine a stick with a string running along its top and a flag waving behind it. The stick is the notochord and the string is the nerve cord. Every chordate starts life looking something like this. People have these parts too, but only for a short time as embryos.

Vertebrate Classes

Vertebrates are chordates with a vertebral column, the backbone, a series of blocks that protects the nerve cord and anchors muscles. Roughly 70,000 vertebrate species exist, a small fraction of the animal kingdom, yet they dominate the oceans, the land, and the air. Zoologists sort them into seven classes, summarized below.

The jawless fishes are the most ancient living vertebrates, with hagfish and lampreys retaining a cartilaginous skeleton and a round mouth without jaws. Chondrichthyes are the sharks, rays, and skates, which have jaws and a skeleton of cartilage rather than bone. Actinopterygii, the ray-finned fishes, dominate today's waters with over 30,000 species and a skeleton of bone. Amphibia are the frogs, salamanders, and caecilians, tetrapods whose moist skin and aquatic eggs tie them to water. Reptilia in the traditional scheme includes snakes, lizards, turtles, and crocodilians, all scaly amniotes. Aves are the birds, feathered endotherms that descend from reptiles. Mammalia includes humans and is defined by hair and by mammary glands that produce milk for the young. Birds and mammals keep a steady body temperature, while reptiles, amphibians, and fishes warm up or cool down with their surroundings.

Common Mistake: Amphibians are not fully terrestrial animals. Their skin and eggs still need moisture, so frogs must return to water to breed. That is why the amniotic egg matters so much for reptiles, birds, and mammals.

ELI-10

Think of the vertebrate classes as teams in a sports league. Each team wears a uniform you can spot at a glance. Scales, feathers, or hair are those uniforms. When you name the uniform, you usually know the class.

Key Evolutionary Innovations

Vertebrate history reads as a series of evolutionary innovations, new features that opened fresh habitats and lifestyles. Jaws evolved from the first gill arches and let animals bite, tear, and swallow much larger food. Paired limbs carried vertebrates onto land. The amniotic egg was the decisive innovation for land, because its internal membranes and shell protect the embryo from drying out, freeing reptiles, birds, and mammals from the need to breed in water. Endothermy, the internal generation of body heat, allowed birds and mammals to stay active in cold conditions, powered by high metabolic rates. Hair and feathers then insulated those warm bodies, and lactation let mothers feed their young without foraging for each meal. Each innovation built on the one before it, which is why the innovations map cleanly onto the classes: jaws in fishes, limbs in tetrapods, the amniotic egg in reptiles, birds, and mammals, and endothermy in birds and mammals.

Common Mistake: An amniotic egg does not have to be hard-shelled. Many reptile eggs are leathery and some stay soft. The protective membranes inside, not the shell, are what free the embryo from water.

ELI-10

Picture a key ring where each key opens a new door. Jaws opened the door to bigger meals. The amniotic egg opened the door to dry land. Feathers and hair opened the door to cold weather. Each key is an innovation that unlocked a new way of life.

High-Yield:

• Sort animals by symmetry, germ layers, and body cavity before naming a phylum.
• All chordates show a notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail at some stage.
• Protostomes build the mouth first, deuterostomes the anus first, and humans are deuterostomes.
• Arthropods are the largest phylum, and echinoderms are the invertebrates closest to chordates.
• Jawless fishes, cartilaginous fishes, ray-finned fishes, amphibians, reptiles, birds, and mammals each carry one defining innovation forward.

Quick Review

• Radial symmetry suits sessile or drifting animals, while bilateral symmetry pairs with cephalization.
• Acoelomates have no body cavity, pseudocoelomates have one lined on a single side, and coelomates have a fully lined cavity.
• Protostomes and deuterostomes differ in which opening forms first during development.
• The major invertebrate phyla are Porifera, Cnidaria, Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, and Echinodermata.
• The four chordate features are the notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail.
• Vertebrate classes run from jawless fishes through cartilaginous and ray-finned fishes, amphibians, reptiles, and birds to mammals.
• Jaws, limbs, the amniotic egg, and endothermy are the landmark innovations of vertebrate history.
• Echinoderms and chordates share deuterostome development, the key link between the two groups.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.