Anatomy of finfish and shellfish

Unit 6 - Respiratory system in fishes

6.2.2. Gill slits
There are six or seven pairs of gills in cartilaginous fishes while four pairs in bony fishes due to the loss of spiracle. Gill slits of bony fishes are covered by operculum while operculum is absent in cartilaginous fishes. In sharks gill slits are situated while in rays they are ventrally placed. A pair of spiracle is present in Elasmobranchii anterior to first gill which corresponds to a vestigial primitive first gill slit. Although spiracle is absent in bony fishes, in Actinopterygii it is replaced by a pseudobranch which is free in some fishes but skin covered in others.