Which phylum was the first to develop segmentation via septa?

Which phylum was the first to develop segmentation via septa?

• A. Annelida
• B. Nematoda
• C. Mollusca
• D. Arthropoda

Answer: (A)

Segmentation via septa means the body is divided into repeating units that are separated by internal walls, so the coelomic cavity is partitioned into multiple compartments. Annelids are the classic examples of this true metamerism: each segment contains its own musculature, nerves, and organs, and the septa physically separate the coelomic spaces from one segment to the next. This arrangement supports independent movement and specialization of each segment, a hallmark feature of earthworms, leeches, and their kin. Nematodes don’t have true segmentation at all, so there aren’t repeated internal compartments separated by septa. Mollusks lack this kind of body segmentation; their body plan isn’t built from a chain of metameric units. Arthropods do show segmentation, but their body cavity is organized differently and isn’t partitioned into many coelomic compartments by septa the way annelids are. So the development of segmentation through septa is a defining trait of annelids.

Segmentation via septa means the body is divided into repeating units that are separated by internal walls, so the coelomic cavity is partitioned into multiple compartments. Annelids are the classic examples of this true metamerism: each segment contains its own musculature, nerves, and organs, and the septa physically separate the coelomic spaces from one segment to the next. This arrangement supports independent movement and specialization of each segment, a hallmark feature of earthworms, leeches, and their kin.

Nematodes don’t have true segmentation at all, so there aren’t repeated internal compartments separated by septa. Mollusks lack this kind of body segmentation; their body plan isn’t built from a chain of metameric units. Arthropods do show segmentation, but their body cavity is organized differently and isn’t partitioned into many coelomic compartments by septa the way annelids are. So the development of segmentation through septa is a defining trait of annelids.