Terminology

• Symmetry: A term used to describe the geometrical axes for the arrangement of body parts in an organism. There are four major symmetrical types: 1) Asymmetrical organisms have no discernible shape. They are "blob-like" in structure and it is not possible to cut them along any plane to produce two halves that are similar; 2) Spherical organisms are shaped like a ball. You can cut them through the center along any number of planes and the two halves will look alike; 3) Organisms with Radial symmetry are cylindrical in shape, often with tentacles at one end. A dandelion is radially symmetrical and can be cut along many planes (like spokes dividing a wheel) to produce two segments that are more-or-less identical in structure; 4) Bilateral organisms can be cut only along one plane to generate to similar halves. For bilateral animals like ourselves, the proper plane would divide us into left and right sides. NOTE: Worms, even though they are cylindrical in shape, have a top/bottom and left/right axes. Worms therefore have bilateral symmetry.
• Dorsal: Referring to the back (or top surface) of an animal.
• Ventral: Referring to the belly (or underside) of an organism.
• Anterior: The front or head end of the animal.
• Posterior: The back or tail end of an animal.
• For radially symmetrical organisms without an anterior or posterior end, the oral surface is the side on which the mouth lies (the aboral surface is the opposite side).
• Segmentation: Strictly speaking, for segmentation to exist the segments must divide the organism internally, and internal structures should repeat over at least a portion of the body. Organisms that simply have grooves in their outer skin are said to be annulated.
• Internal skeletons are composed of bone and are covered with flesh (except for teeth). We have an internal skeleton. An organism is said to possess an external skeleton if an outer covering surrounds soft tissue on the inside. Insects and crabs have an external skeleton (despite the use of the term "soft-shelled crabs," these animals do not have shells). While skeletons have numerous joints to permit a wide range of movements, shells have at most only one. For the purposes of this laboratory, an animal is said to have a shell if the structure is loosely attached and provides no extended support for the organism. Clams have shells. When opened their innards fit loosely inside and are attached to the shell at only a few points.
• Antenna: a sensory structure extending from the head of the animal. Antennae may be long and thin or feather-like in their structure. Some animals have short tufts for antennae (house flies, for example). Appendages that are used to grab or manipulate food and other objects are called tentacles. Cockroaches have antennae; an octopus has tentacles.

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