Invertebrate Zoology Terminology

Use this list as a quick reference to some of the terminology that you will encounter in Invertebrate Zoology. It is important to note that this is not a complete list, so you should add to it as you encounter new terms not listed here.

1. Level of Organization

• Cytoplasmic/protoplasmic – all life activities are carried out within the limits of a single cell membrane
• Cellular – many different functions are carried out by specific cellular structures
• Tissue – similar cells grouped together to perform common functions as a highly coordinated unit
• Organ – tissues are assembled into these larger functional units
• Organ System – different organs operate together to perform vital functions

2. Symmetry

• Asymmetry – The arrangement of body parts without a central axis or point (e.g., sponges)
• Radial – The arrangement of body parts such that any plane passing through the central oral-aboral axis divides the animal into mirror images (e.g., the cnidarians). Radial symmetry can be modified by the arrangement of some structures in pairs, or other combinations, around the central axis (e.g., biradial symmetry in the ctenophorans and some anthozoans), and pentaradial symmetry in the echinoderms.
• Bilateral – The arrangement of body parts such that a single plane passing between the upper and lower surfaces and through the longitudinal axis divides the animal into right and left mirror images (e.g., the vertebrates).

3. Terms of Direction

• Aboral – end opposite mouth
• Oral – end containing the mouth
• Anterior – head end; usually the end of a bilateral animal that meets its environment
• Posterior – tail end
• Caudal – toward the tail
• Cephalic – toward the head
• Distal – away from the point of attachment of a structure on the body (e.g., the toes are distal to the knee)
• Proximal – toward the point of attachment of a structure on the body (e.g., the hip is proximal to the knee)
• Dorsal – the back of an animal; usually the upper surface
• Ventral – the belly of an animal; usually the lower surface
• Inferior – below a point of reference (e.g., the mouth is inferior to the nose in humans)
• Superior – above a point of reference (e.g., the neck is superior to the chest)
• Lateral – away from the plane that divides a bilateral animal into mirror images
• Medial (median) – on or near the plane that divides a bilateral animal into mirror images

4. Germ Layers

• Diploblastic – simplest level of tissue organization, consisting of two embryological layers: (1) ectoderm which gives rise to the epidermis (outer layer of the body wall) and (2) endoderm which gives rise to the gastrodermis (tissue that lines the gut cavity)
• Triploblastic – more complex level of tissue organization, consisting of three embryological layers: (1) ectoderm; (2) endoderm and (3) mesoderm (between ectoderm and endoderm; gives rise to the organs of circulation, excretion, and reproduction plus the muscles, connective tissue, and blood cells.)
• It is important to remember that sponges have no true tissues and cnidarians are diploblastic; however most animals are triploblastic. Within the triploblastic groups, different developmental patterns have arisen. The three patterns are discussed below.

5. Body Cavities
Three different ways of compartmentalizing the body space have evolved: acoelomate, pseudocoelomate, and eucoelomate.

Coelom: A fluid-filled cavity between the body wall and the gut which is lined by mesoderm (coelom formation).

o Advantages:
o Provide more room for organ development
o Provide more surface area for diffusion of gases, nutrients and wastes into and out of organs
o Provide an area for storage
o Often acts as hydrostatic skeleton
o Provide a vehicle for eliminating wastes and reproductive products from the body
o Facilitate increased body size

Reminder: the gut (digestive tract) is not a body cavity.

• Acoelomate – “without” body cavity surrounding the gut; the region between the epidermis and the digestive tract is filled with parenchyma (parenchymal cells are not specialized for a particular function); gut is the only internal space.
• Pseudocoelomate – a “false” body cavity surrounds the gut but lacks a peritoneal lining; pseudocoelom derived from the blastocoel of the embryo; two internal spaces: a persistent blastocoel (pseudocoelom) and a gut.
• Eucoelomate – a “true” body cavity with a peritoneal lining; two internal spaces, a gut and a coelom. The body cavity is lined with mesoderm.
• Schizocoel - found in molluscs, annelids, and arthropods; coelom derived from the mesodermal layer by splitting
• Enterocoel - found in echinoderms, hemichordates and chordates; coelom arises from outpouching of the mesodermal layer in the archenteron

6. Protostome/Deuterostome

The coelomates have been traditionally been divided into two large groups, and many of the reasons for this separation come from comparative embryology. Within the last ten years, molecular data has caused systematist to reevaluate the interpretation of traditional groupings. The protostomes may not be a valid taxonomic group – ie, not monophyletic – and the animal phyla that it contained are often divided into two large groups, the Ecdysozoa and the Lophotrochozoa. Zoologists continue to view deutersomes as a true evolutionary lineage.

• Protostome - embryonic development that characterizes molluscs, annelids, and arthropods; includes four main components:

· Spiral Cleavage

· Blastopore develops into the mouth (proto = first, stome = mouth)

· Schizocoelous

· Determinate (Mosaic) Embryo – removing a cell from the 4-cell stage results in arrested development of the embryo

• Deuterostome – embryonic development that characterizes echinoderms and chordates. Includes four main components that differ from protostomes.

· Radial Cleavage

· Blastopore develops into the anus (deutero = second, stome = mouth)

· Enterocoelous

· Indeterminate (Regulative) Embryo – removing a cell from an embryo does not affect development

7. Skeletons

Animal skeletal systems provide physical support for the body and protection for the soft tissues. They also act as the support necessary for antagonistic muscle action (ie, a framework against which muscles work to move parts of the body or the entire organism). There are three main types of skeletons found in both invertebrates and vertebrates: hydrostatic skeletons, exoskeletons, and endoskeletons.

A hydrostatic skeleton is a structure consisting of muscles and fluid that, by themselves, provide support for the animal; no rigid support, like bone, is involved. A hydrostatic skeleton consists of a body compartment or compartments filled with water or body fluids, which are incompressible liquids. When the muscular walls of the compartment contract, they pressurize the contained fluid. The contractions and relaxations of the muscles surrounding the compartments change the shape of the animal. Hydrostatic skeletons are the primary support systems of cnidarians, flatworms, roundworms, and annelids. In all these animals, compartments containing fluids under pressure make the body semi-rigid and provide a mechanical support on which muscles act.

An exoskeleton is a rigid external body covering, such as a shell, that provides support. In an exoskeleton, the force of muscle contraction is applied against that covering. An exoskeleton also protects delicate internal tissues such as the brain and respiratory organs. Many molluscs, such as clams and oysters, have an exoskeleton consisting of the hard calcium carbonate shell secreted by glands in the mantle. Arthropods, such as insects, spiders, and crustaceans, have an external skeleton in the form of a chitinous cuticle, secreted by underlying tissue that covers the outside surfaces of the animal. The exoskeleton protects against dehydration, serves as armor againsts predators, and provides the levers against which muscles work. In many flying insects, elastic flexing of the exoskeleton contributes to the movements of the wings.

An endoskeleton consists of internal body structures that provide support. Like exoskeletons, endoskeletons also protect delicate internal tissues. Echinoderms have an endoskeleton consisting of ossicles (ossiculum = little bone), formed from calcium carbonate crystals. The shells of sand dollars and sea urchins are the endoskeletons of these animals.

8.   Summing it all up

     Over 99% of the animal species living today are bilaterally symmetrical triploblasts with coeloms and follow either the protostome or deuterostome pattern of development. This combination of features has proven to a very successful way to organize a moving and eating machine. The basic animal body is a tube-within-a-tube. The inner tube is the gut, which is derived from endoderm. The outer tube forms the epidermis or skin, which is derived from ectoderm. The mesoderm in between forms muscles and organs.

     What about more complex-looking animals, such as grasshoppers, and crayfish and horses? The body plan of these animals can also be thought of as a tube-within-a-tube, but mounted on legs. Keep in mind that most animals with complex-looking bodies are relatively long and thin. They have an outer body wall that is more or less tubelke and an inner gut that runs from mouth to anus. The body cavity itself is filled with muscles and organs derived from mesoderm. Legs and wings are just efficient ways to move a tube-within-a-tube around the environment. Once the basic tube-within-a-tube evolved, most of the diversification of animals was characterized by the evolution of novel types of structures for moving and capturing food.

9. Nutrition

o Autotrophic – self nourishing; uses carbon dioxide as its source of carbon and makes its organic nutrients from inorganic raw materials

o Heterotrophic – obtains energy from organic compounds by consumption of other organisms

o Saprozoic – nutrition by absorption of simple organic nutrients from surrounding medium

Some information for this page was obtained from: Miller S. Zoology 6^th Edition, New York: McGraw-Hill, 2005.

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