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

2.      Symmetry

3.      Terms of Direction

4.      Germ Layers

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.

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.

·        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

·        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 6th Edition, New York: McGraw-Hill, 2005.

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