It's the opening of a tube, called the spiracular tract, that's used to bring water in toward the gills.

这是一根管子的开口， 称为孔道，用于将水带入鳃中。

In Eusthenopteron, the spiracular tract opens up near the front of the skull, just like in most water-breathing fish with gills.

在 Eusthenopteron 中， 孔在头骨前部附近打开，就像大多数带鳃的水肺鱼一样。

And by the time the Devonian Period had ended and the Carboniferous was underway, they had also lost their spiracular tracts, and started using a totally different kind of skull-hole for breathing, called nostrils.

到泥束和石炭开始时，他们也失去了孔，并开始使用一种完全不同的颅骨孔来呼吸，称为鼻孔。