Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Symmetry, Segmentation and Body Planes.

You, a Hydra and a sponge: try to cut each of you into two equal halves. You can be split in only one way, the Hydra in more than one way, and the sponge in no way at all. How many equal parts a body gives, and along which plane or axis, is what symmetry is about, and this page starts right there.

1. The five kinds of symmetry

Symmetry is the position of similar parts of equal shape and size on the two sides of the median plane of the body. If an animal can be divided into equal parts along some axis or plane it is symmetrical; if it cannot, it is asymmetrical.

In spherical symmetry the body divides into similar parts along any plane through its centre, just like a ball. Examples are Volvox, Radiolaria and Heliozoa, all single-celled protists.

In radial symmetry a cut along the central vertical axis divides the body into more than two similar halves, with the mouth at one end of the long axis. Examples are Hydra (Hydra vulgaris), Aurelia and Metridium.

In biradial symmetry only two planes, at right angles to each other, can pass along the long axis, so the body divides into 4 similar parts. An example is Coeloplana of phylum Ctenophora.

In bilateral symmetry the body divides only once, along the sagittal plane, into similar right and left halves. Examples are the butterfly (Pieris brassicae), the frog (Fejervarya asmati) and the human.

In asymmetry no axis or plane gives two similar parts, not even once. Examples are the sponge (Cliona celata) and the apple snail (Pila globosa).

Symmetry is whether a cut gives matching halves. Turn the plane and cut.

Now sort the animals by symmetry.

2. Segmentation and tagmatization

When an animal's body is built from the repetition of similar segments along its long axis, we call it segmentation or metamerism. Each segment is a metamere or somite.

There are three kinds, depending on how the segments look. Alike segments make homonomous segmentation, as in the earthworm. Unlike segments make heteronomous segmentation, as in insects. No segments at all makes an asegmental animal, such as the starfish and the mussel.

Segmentation and tagmatization are not the same thing. When some segments of an arthropod's body fuse into a few definite regions, the tagmata, this is tagmatization. A grasshopper has three tagmata: head, thorax and abdomen. Tagmatization counts when Arthropoda are classified.

3. Ends and planes

In a bilaterally symmetrical animal the end with the mouth is the head, and the opposite end is the anal or tail end. This difference between the two ends is called polarity.

There are five kinds of polarity: anterior, posterior, dorsal, ventral and lateral. The anterior end bears the head and the posterior end is opposite it. The dorsal end is the upper surface, the ventral end the lower surface, and the lateral end the two sides.

Three kinds of plane are usually seen in the animal body. The median (sagittal) plane divides the body into similar right and left halves. The frontal plane divides it along the long axis into dorsal and ventral parts. The transverse plane divides it, at right angles to the median plane, into anterior and posterior halves. Now find all eight places on the rabbit of Figure 1.8.

Now turn the cards over and test yourself.

4. Check yourself

Three questions in the exam's style. Start with the first.

In the second, count the parts.

In the last, think of planes and ends together.

The whole page on a few cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Symmetry, Segmentation and Body Planes” part.