Final answer:
Animals with a segmented body plan have their bodies divided into repetitive parts or segments, contributing to more complex organ systems. Annelids, like earthworms and leeches.
Step-by-step explanation:
Segmented Body Plan in Animals
Animals with a segmented body plan exhibit a division of their body into multiple parts called segments. This body organization can be observed in phyla such as Annelida and Arthropoda. Annelids, for instance, include animals like earthworms and leeches that demonstrate a metameric form of segmentation, meaning each segment harbors repeated internal and external structures. These structures contribute to well-developed organ systems such as the neuronal, circulatory, and digestive systems. Annelids can be further divided into polychaetes—characterized by parapodia and bristles—and oligochaetes, such as earthworms, which lack parapodia and may have fewer or no bristles at all.
The presence of a fluid-filled body cavity is also a hallmark of segmented worms, acting as a hydroskeleton that aids in muscle movement. Notably, the more sophisticated organ systems are apparent when compared to non-segmented worms like flatworms. For example, earthworms have a complete digestive tract and a circulatory system with five pairs of hearts and two main vessels. Their nervous system encompasses a simple brain connected to a ventral nerve cord.
In terms of broad classification, animals are distinguished by their body symmetry, the presence of three germ layers, and whether they have an internal body cavity (coelom). Among coelomates, organisms can then be grouped into protostomes or deuterostomes based on embryological development stages.