Which phylum is characterized by a ventral nerve cord and anterior ganglia?

Which phylum is characterized by a ventral nerve cord and anterior ganglia?

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

Answer: (A)

Think about how the nervous system is organized along the body. A ventral nerve cord running along the belly, paired with an anterior pair of ganglia acting as a simple brain, is a distinctive setup of segmented worms. In this phylum, the nervous system is clearly arranged with a brain-like ganglion at the head and a ventral nerve cord that extends across most of the body, with a ganglion in each body segment helping coordinate movement and function. Other phyla have different nervous layouts. Rotifers have a nervous system that is more centralized near the head with a nerve ring and some longitudinal cords but not a prominent, continuous ventral nerve cord with segmental ganglia. Nematodes possess a nerve ring around the pharynx plus longitudinal nerve cords, but they lack the clearly segmented ventral cord with per-segment ganglia. Molluscs show a variety of arrangements, including a brain and paired nerve cords, but not the characteristic ventral nerve cord with anterior segmental ganglia seen in annelids. So the combination of a ventral nerve cord and anterior ganglia points to the segmented worms: annelids.

Think about how the nervous system is organized along the body. A ventral nerve cord running along the belly, paired with an anterior pair of ganglia acting as a simple brain, is a distinctive setup of segmented worms. In this phylum, the nervous system is clearly arranged with a brain-like ganglion at the head and a ventral nerve cord that extends across most of the body, with a ganglion in each body segment helping coordinate movement and function.

Other phyla have different nervous layouts. Rotifers have a nervous system that is more centralized near the head with a nerve ring and some longitudinal cords but not a prominent, continuous ventral nerve cord with segmental ganglia. Nematodes possess a nerve ring around the pharynx plus longitudinal nerve cords, but they lack the clearly segmented ventral cord with per-segment ganglia. Molluscs show a variety of arrangements, including a brain and paired nerve cords, but not the characteristic ventral nerve cord with anterior segmental ganglia seen in annelids.

So the combination of a ventral nerve cord and anterior ganglia points to the segmented worms: annelids.