Which phylum would you classify an organism with a closed circulatory system, a ventral nerve cord, and a digestive tract with a gizzard?

Which phylum would you classify an organism with a closed circulatory system, a ventral nerve cord, and a digestive tract with a gizzard?

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

Answer: (A)

A good way to read this combination is to focus on how body plan and organ specialization fit together in certain groups. A closed circulatory system means blood stays in vessels, which is a feature of segmented worms. The ventral nerve cord running along the underside with segmental nerve tissue is another hallmark of these animals. And having a digestive tract with a gizzard—an enlarged muscular stomach that grinds up food—is a classic feature seen in earthworm-type annelids. Put together, these traits strongly point to annelids. Rotifers and nematodes lack a true closed circulatory system and a continuous ventral nerve cord, and their digestive tracts don’t include a grinding gizzard. Mollusks mostly have an open circulatory system, and their nervous and digestive arrangements differ from this combination.

A good way to read this combination is to focus on how body plan and organ specialization fit together in certain groups. A closed circulatory system means blood stays in vessels, which is a feature of segmented worms. The ventral nerve cord running along the underside with segmental nerve tissue is another hallmark of these animals. And having a digestive tract with a gizzard—an enlarged muscular stomach that grinds up food—is a classic feature seen in earthworm-type annelids. Put together, these traits strongly point to annelids.

Rotifers and nematodes lack a true closed circulatory system and a continuous ventral nerve cord, and their digestive tracts don’t include a grinding gizzard. Mollusks mostly have an open circulatory system, and their nervous and digestive arrangements differ from this combination.