Most morphological diversity in animals is based on differences in mouths and limbs, which are used for eating and moving.
However, the basic architecture of the animal body has been highly conserved throughout evolution.
Four key features define an animal's body plan:
Number of embryonic tissue layers
Type of body symmetry and degree of cephalization (formation of a head region)
Presence or absence of a fluid-filled body cavity
How the earliest events of embryonic development proceed
Evolution of Animal Tissues
All animals have tissues, which are tightly integrated structural and functional units of cells.
All animals have epithelium, a layer of tightly joined cells that covers the body surface
Diploblasts are animals with two embryonic tissue layers:
Ectoderm ("outside skin")
Endoderm ("inside skin")
Triploblasts are animals with three embryonic tissue layers:
Ectoderm
Endoderm
Mesoderm ("middle skin") which develops into distinct adult tissues and organs.
Symmetry and Cephalization
A basic feature of a multicellular body is the presence or absence of a plane of symmetry.
Most sponges are asymmetrical.
Animals with radial symmetry have at least two planes of symmetry.
Organisms with bilateral symmetry have a single plane of symmetry and face their environment in one direction.
Bilateral symmetry allowed cephalization, the development of a head region where structures for feeding, sensing the environment, and processing information are concentrated.
Evolution of a Body Cavity
Animals may or may not have an enclosed, fluid-filled body cavity called a coelom.
Triploblasts that do not have a coelom are called acoelomates; those that possess a coelom are coelomates.
The coelom forms from within the mesoderm and thus is lined on both sides with cells from the mesoderm.
The coelom creates a container for circulation of oxygen and nutrients, and also acts as an efficient hydrostatic skeleton that allows soft-bodied animals to move even without fins or limbs.
Acoelomates, Coelomates, and Pseudocoelomates
Acoelomates: have no enclosed body cavity.
Coelomates: have an enclosed body cavity completely lined with mesoderm.
Pseudocoelomates: have an enclosed body cavity partially lined with mesoderm.
Hydrostatic Skeleton
Hydrostatic skeletons allow limbless animals to move.
Mechanism:
Fluid-filled pseudocoelom under pressure creates tension in the body wall.
Body wall is in tension, creating pressure in the fluid.
Muscles cause shape change.
Coordinated muscle contractions result in locomotion; when muscles on one side contract, the fluid-filled chamber changes shape, and the animal bends.
Patterns of Development
Except for adult echinoderms, all coelomates (including juvenile echinoderms) are bilaterally symmetric and have three embryonic tissue layers.
This group, called the Bilateria, can be divided into protostomes (arthropods, mollusks, and segmented worms) and deuterostomes (chordates and echinoderms).
Protostome and Deuterostome Patterns of Development
Three events in embryonic development differ in protostomes and deuterostomes:
Cleavage
Gastrulation
Coelom formation
Cleavage: a rapid series of mitotic divisions that occurs in the absence of growth.
Gastrulation: a series of cell movements that results in the embryonic tissue layers; as gastrulation proceeds, the coelom forms.
Cleavage
Protostomes: Spiral cleavage (cells rotate so they are not directly on top of each other).
Deuterostomes: Radial cleavage (cells stack on top of each other).
Gastrulation and Coelom Formation
Protostomes: Pore becomes mouth.
Deuterostomes: Pore becomes anus.
Coelom formation:
Protostomes: Block of solid mesoderm splits to form coelom.
Deuterostomes: Mesoderm pockets pinch off of gut to form coelom.
Molecular Phylogenies
After the split between the protostomes and the deuterostomes, the protostomes split to form two major subgroups: the Ecdysozoa and the Lophotrochozoa.
Segmentation evolved independently in annelids and arthropods, as well as in vertebrates and possibly in molluscs.
Vertebrates, those animals with skulls and usually backbones, are a monophyletic lineage. Invertebrates, all animals that are not vertebrates, are a paraphyletic group.
Feeding
Suspension (or filter) feeders: capture food by filtering out particles suspended in water or air.
Deposit feeders: eat their way through a substrate.
Fluid feeders: suck or mop up liquids like nectar, plant sap, blood, or fruit juice.
Mass feeders: take chunks of food into their mouths.
What Animals Eat
Animals that feed on plants or algae are classed as herbivores; those that feed on other animals are carnivores; and those that feed on dead organic matter are detritivores. Omnivores, such as humans, eat both plants and animals.
Herbivores and carnivores can be further classified as either predators or parasites.
Predators kill other organisms for food using an array of mouthparts and hunting strategies.
Parasites take nutrients from living animals. Endoparasites live inside their hosts. Ectoparasites live outside their hosts.
Reproduction and Birth
Embryos may be retained in the female's body during development (viviparous), or eggs may be laid outside to develop independently of the mother (oviparous).
Some species are ovoviviparous. In such species, the female retains eggs inside her body during early development, but the growing embryos are nourished by yolk inside the egg and not by nutrients transferred directly from the mother.
Life Cycles
Animal life cycles vary widely and may include metamorphosis.
Metamorphosis is the change from juvenile to adult body type.
Holometabolous metamorphosis (complete metamorphosis): The juvenile individual is called a larva and looks quite different from the adult form. When the larva has grown enough, it encases itself and becomes a pupa; in this stage, it is remodeled into an adult.
Hemimetabolous metamorphosis (incomplete metamorphosis): The juvenile form is called a nymph and looks like a miniature adult.
Metamorphosis Examples
Complete metamorphosis is common in insects and marine animals.
For example, some cnidarians have two distinct body types during their life cycle: a largely sessile form called a polyp alternates with a free-floating stage called a medusa.
Polyps and medusae live in different habitats, and the two stages of the life cycle exploit different food sources.
Basal Animal Groups
Phylogenetic analyses and the fossil record indicate several basal (most ancient) animals:
Porifera (sponges)
Cnidaria (jellyfish and others)
Ctenophora (comb jellyfish)
Acoelomorpha
Porifera (Sponges)
About 7000 species of sponges have been described to date.
Benthic: live at the bottom of aquatic environments.
Their body architecture is built around a system of tubes and pores that create channels for water currents.