Cracking the AP Biology Exam


Diversity of Organisms


Animals are multicellular, heterotrophic eukaryotes that are extremely diverse. Some of the questions on the AP Biology exam will focus on the major features of animal body plans.


One way to categorize animals is according to their body symmetry. Most animals have some form of symmetry, either radial symmetry or bilateral symmetry. An animal with radial symmetry has a top side and bottom side but no left or right side. These animals are easily identified by their circular body pattern. An example is a sea anemone. An organism with bilateral symmetry has a top side (dorsal), a bottom side (ventral), as well as a left and right side. They also have a head end (anterior) and tail end (posterior).


Animals with bilateral symmetry have a tendency for sensory and neural organs to be concentrated at the anterior end of the body, leading to the development of the brain, a process known as cephalization.


Animal body plans can differ according to tissue complexity. Some animals, such as cnidarians, are diploblastic (have two germ layers). The two germ layers are the ectoderm and the endoderm. Other animals, such as tapeworms, are triploblastic (have three germ layers)—the ectoderm, the mesoderm, and the endoderm.


Some triploblastic animals have a body cavity. Early animals lacking a body cavity are known as acoelomates. An example is a flatworm. Other animals developed a body cavity derived from the mesoderm and are known as coelomates. A coelom cushions the internal organs and protects them from injury. Annelids are the first coelomates. A third group of animals develop a body cavity that is derived from the blastocoel rather than the mesoderm. These animals, such as nematodes, are known as the pseudocoelomates.

Protostomes and Deuterostomes

Coelomates can be categorized according to differences in the pattern of early development. There are two developmental patterns—protostome development and deuterostome development—based on the pattern of cleavage of the zygote. Animals with protostome development exhibit spiral cleavage, which is determinate, meaning that the fate of each embryonic cell is fixed very early. For example, at the four-cell stage each embryonic cell can only develop into a fixed quarter of the embryo. Furthermore, in protostomes, the first opening in development, the blastopore, later forms an internal cavity called the archenteron. The first infolding of the archenteron becomes the animal’s mouth and the mesoderm tissue splits and forms the coelom.

Animals with deuterostome development exhibit indeterminate cleavage, meaning each cell is capable of forming a complete embryo. In deuterosomes, the blastopore becomes the animal’s anus. In deuterostomes, the first infolding of the archenteron becomes the animal’s anus and the coelom forms from mesodermal outpocketings of the archenteron.

What phyla belong to the kingdom Animalia? Check out the list below.

A. Porifera

Sponges are sessile (nonmotile) organisms whose bodies are constructed of two cell layers. They ingest food by drawing a steady current of water through their pores. Sponges have radial symmetry and are acoelomates (have no body cavity).

B. Cnidaria

Cnidarians have a two-cell layered body with a digestive cavity surrounded by tentacles that sting their prey. They are acoelomates. Common cnidarians are jellyfish, hydras, and sea anemones.

C. Platyhelminthes

Flatworms are motile organisms whose bodies are the first to have three cell layers and bilateral symmetry. They are also acoelomates (no body cavity). Platyhelminthes can be parasitic, and can undergo regeneration.

D. Nematoda

Roundworms are soil-dwellers that have pseudocoelomate bodies. They have a complete digestive tract that extends from mouth to anus. Some are parasites.

E. Rotifera

Rotifers are tiny filter feeders that are pseudocoelomates with a complex, complete digestive system.

F. Mollusca

Mollusks are motile organisms with soft bodies and hard shells. They are the first protostomes and the first coelomates (with spiral, determinate cleavage). Members of the mollusks include octopuses, squids, snails, and clams.

G. Annelida

Annelids are segmented worms with two openings: a mouth and anus. They have a fully developed digestive system, a closed circulatory system, a developed nervous system, and bristle appendages (setae). The most common examples are earthworms.

H. Arthropoda

Arthropods are animals with segmented bodies; paired, jointed legs; and a chitinous exoskeleton. They are unusual in that they have an open circulatory system with a dorsal heart. Examples include insects, arachnids, and crustaceans.

I. Echinodermata

Echinoderms are sessile or sedentary animals with a spiny exoskeleton. They are the first deuterosomes (the blatospore develops into the anus). An example of an echinodermata is a sea urchin.

J. Chordata

Chordates are animals that have a notochord, a dorsal nerve cord, a postanal tail, and pharyngeal gill slits at some time in their lives. Although most are vertebrates (have backbones), some are not. The invertebrates include tunicates, amphioxus, and acorn worms.

i. Pisces
Fish are cold-blooded vertebrates that have gills, scales, and a two-chambered heart.

ii. Amphibia
Amphibians initially breathe through gills, and then develop lungs. They can also exchange gas through their moist skin. They have a three-chambered heart.

iii. Reptilia
Reptiles are cold-blooded animals that have eggs with a chitinous covering. They have a four-chambered heart and are the first vertebrates to have internal fertilization.

iv. Aves
Birds are warm-blooded, have eggs with shells, wings, feathers, hollow bones, and a four-chambered heart.

v. Mammalia
Mammals are warm-blooded animals with a four-chambered heart. They have hair and produce milk to feed their young. Some mammals have a placenta (a structure that nourishes the fetus). Marsupials, on the other hand, do not have a placenta, and the developing embryo receives very little nourishment from the mother in the uterus. About eight days after fertilization, the fetus must continue its maturation in its mother’s pouch.