gill slit
The external opening of a fish's gill chamber; a standard anatomical term.
The shark's gill slits are lined up on the side of its body.
pharyngeal slit
In vertebrate embryology, a structure homologous to gill slits; more technical than sense 1.
In embryology, pharyngeal slits are sometimes observed.
Standard kanji spelling for gill slit; 鰓 means gill and 孔 means hole.
Kana spelling is common in everyday writing, especially when the kanji 鰓 is considered difficult.
Compound of 鰓 (えら, 'gill') and 孔 (あな, 'hole'). The reading さいこう is the on'yomi reading of the kanji, used in technical contexts.