Segmentation and Notochord

Two further structural features complete the basis-of-classification toolkit.

Segmentation (metamerism) is the division of the body, both externally and internally, into a series of repeated units called segments, often with a serial repetition of organs. The simplest form is seen in annelids, where each segment closely resembles the next and is internally separated from its neighbours by partitions called septa. In arthropods such as the cockroach, segmentation is retained but has become specialised — different segments look different and perform different functions (grouped into head, thorax and abdomen), reflecting a more advanced, differentiated form of the same underlying plan. …