My textbook says arthropods have a double, ventral, solid nerve cord. And chordates have a single, dorsal, hollow nerve cord.
What is hollow exactly in a cord that looks like this?
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Your book is talking about an embryonic structure, the neural tube. This structure forms from an invagination of the neural plate, a collection of cells on the dorsal surface of the early embryo. The process forms a hollow neural tube that will eventually develop not only into the spinal cord but also into the entire CNS.
There is still a lot of development to happen at this point: the cells making up the neural tube will divide many times and proliferate to create the central nervous system. Within the section that becomes the spinal cord, hollow part remains and becomes the central canal, contiguous with the fourth ventricle in the brain and containing cerebro-spinal fluid:
(note: I had a bit of trouble finding an image showing stages of spinal cord development with a license suitable for posting here, but you can find a bunch if you image search for "spinal cord development". Some examples: https://www.medicalimages.com/stock-photo-illustration-of-spinal-cord-development-within-the-embryo-atweeks-five-a-eight-b-and-ten-c-image22427030.html https://www.brainkart.com/article/Development-of-the-Spinal-Cord_18920/ )