neuropores

The nervous system is thus essentially epidermal in position and diffuse in distribution; but an interesting concentration of nerve-cells and fibres has taken place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores.

Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 2 "Baconthorpe" to "Bankruptcy" Various

Sometimes the central canal is wide and uninterrupted between the two neuropores; in other cases it becomes broken up into a large number of small closed medullary cavities, and in others again it is obsolete.

Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 2 "Baconthorpe" to "Bankruptcy" Various