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Chapter 18: 1-10 Simple Invertebrates

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Animals: Classification by Body Plan

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The Classification of Animals

We identify animals as multi-cellular organisms which draw their carbon from organic sources (not from the air, as plants do), and their energy from organic sources as well. As in all other areas of biology, classification of animals is undergoing a revision as we identify similarities in DNA, RNA, and proteins used in similar ways. However, the starting point for classification is still the form and structure of the animal.

General Characteristics and Body Structure

Animals differ from other organisms in several important ways.

Biologists depend heavily on body shape and structure in order to classify animals. We have special terms which we use to refer to the symmetry, or ability to produce a mirror image along some axis or line draw through the body. Radially symmetric animals have a body plan which produces a mirror image when any of several lines are drawn through the center of the body. A body which is symmetrical around only one axis through the middle of the body is bilaterally symmetrical. We can still divide the body into several regions for identification purposes. The dorsal region refers to the backside of the animal, the ventral region to its belly-side. The anterior region is the "front", which may also include the cephalic or head region. The posterior is opposite the anterior region and often includes the tail or caudal section of the animal. The sides are the lateral regions.


Early development

All animals begin as single-celled zygotes. The zygotes divide into more and more cells, which form a hollow sphere or blastula. The cells of the blastula can remain neatly aligned, or they can shift to form a spiral pattern. Animals with aligned blastula usually become deuterstomes; animals with spiral cleavage patterns become protostomes. The blastula eventual forms a pocket (gastrula phase) and in deuterstomes, forms additional pockets that usually lead to animals with coelom. In protostomes, the coelom cavity may or may not form. Most animals with a simple body plan are protostomes, formed from the spiral cleavage of the blastula.

The following animal classification system is based on dividing animals into groups by their body structure—whether or not they possess a coelom, and then by other structural or functional characteristics. Biologists tend to do comparisons that focus on the complexity of particular systems, so it is interesting in the groups below to note the components and organization of the digestive, reproductive, nervous, respiratory, and circulatory systems.


Animals without a coelom

body plan2

or with a pseudocoelombody plan 3

Simple body plans; Porifera and Cnidarians are radially symmetric. Primitive nervous systems and sensory organs, simple to complex digestive and reproductive systems. No skeletal structure.


Porifera (sponges)

  • Types: spiky, glass, and silicon sponges.
  • Habitat: Freshwater and marine habitats.
  • General characteristics: No true organ systems of any kind; multicellular with loose organization. Radial symmetry.
  • Digestion: Nutrients absorbed by surface cells pass through cell membranes directly to internal cells.
  • Reproduction: Some cells produce sperm or eggs for sexual reproduction under adverse circumstances, but most reproduction by budding.
  • Circulatory: No special organs. Direct diffusion among cells.


  • Types: Hydra, jellyfish, corals.
  • Habitat: nearly all marine [there are freshwater hydra].
  • General characteristics: All possess stinging cells (cnidocytes); many occur in one or both polyp and medusa forms. Some organisms (Portuguese man-of-war) are colonies of individual hydra. Radial symmetry; specialized tissues.
  • Digestion: Central cavity with single opening.
  • Reproduction: Simple ovary produces egg cells; also produces asexually.
  • Nervous: No "system" as such. Individual never cells connect sensory cells by irregular nerve nets.
  • Circulatory: No special organs. Direct diffusion among cells.

Ctenophora (comb jellies)

About 100 marine species that are really beautiful. Most have tentacles but lack stinging cells. Not much is known about these deep-water organisms.


Platyhelminthes (flatworms)

  • Types: Planarians, flukes, tapeworms.
  • Habitat: Flatworms are generally free living in marine habitats; the rest are parasites.
  • General characteristics: bilateral symmetry with a head end (cephalization); three discrete tissue layers and organs.
  • Digestion: Separate digestive organs, but only one opening.
  • Reproduction: complex organ system; sexual reproduction; asexual reproduction by binary fission.
  • Nervous: Ganglia and simple brain (massed nervous tissue), sensory organs like eyespots.
  • Circulatory: No special organs. Direct diffusion among cells.


  • Types: Ribbon worms
  • Habitat: Marine with few freshwater and damp soil species.
  • General characteristics: Narrow flattened body. Proboscis is long tube which can be ejected from inside the body to grab prey. Has tube-within-tube body plan.
  • Digestion: Complete tube with two openings (mouth and anus)
  • Reproduction: Complex reproductive organs for sexual reproduction; als asexually by fragmentation.
  • Nervous: Ganglia and simple brain, eyespots.
  • Circulatory: Simple blood vessels, no heart.

Nematodes (roundworms)

  • Types: Roundworms, hookworms, pinworms.
  • Habitat: Widely distributed in soil, marine, and freshwater habitats.
  • General characteristics: Mainly microscopic, with long, thread-like bodies; important in decomposition of organic matter.
  • Digestion: complex tract with (mouth and anus).
  • Reproduction: Sexual
  • Nervous: Simple brain, two nerve cords (dorsal and ventral), simple sense organs.
  • Circulatory: No special organs. Direct diffusion among cells.


  • Types: "wheel" animals, mostly very small marine animals
  • Habitat: Marine or freshwater.
  • General characteristics: Microscopic, with a "crown" of cilia at the tail end that beats during feeding, looking like spinning wheel. Each species has a fixed number of cells arranged in a precise pattern; there is no cell division or growth (or repair) after rotifers complete embryonic development!
  • Digestion: Complete digestive tract.
  • Reproduction: Fission.
  • Nervous: Brain and sense organs, including eyespots.
  • Circulatory: No special organs. Direct diffusion among cells.

At this point, differences become very difficult to track, so the following summeries are more general. See Web Lecture Invertebrates with Coelom.