EVERYDAY SCIENCE

Detailed explanation-1: -The Annelida body plan contains three main body regions: 1) the prostomium, or the head, 2) the trunk, or main body, and 3) the pygidium, the anus. The prostomium is located at the most anterior (forward) region of the worm containing most of the sensory organs.

Detailed explanation-2: -Annelids have a simple brain organ consisting of a pair of nerve clusters in the head region (Fig. 3.49). Nerves link the brain to sensory organs in the head that detect the environment in front of the worm.

Detailed explanation-3: -A major invertebrate phylum of the animal kingdom, the annelids number more than 9, 000 species distributed among three classes: the marine worms (Polychaeta), which are divided into free-moving and sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and the leeches (Hirudinea).

Detailed explanation-4: -The pygidium is the posterior, or back, end of the worm and houses the anus. Annelids grow by adding segments to the region just in front of, or anterior to, the pygidium. Segmented worms are also capable of regrowing regions of the worm that are broken off.

Detailed explanation-5: -Besides the earthworm, the segmented worms also include leeches and some marine worms.