2. Nares: Two crescentic apertures, the nares or nostrils are present ventro-laterally and anterior to mouth.
They are exclusively olfactory, have no role in respiration, as they are not connected to mouth cavity by internal nostrils. 3. External gill slits: Anterior to each pectoral fin, on either side of the body vertically elongated external gill slits or branchial clefts are present in a series of 1 to 5. They are main respiratory organs.
4. Cloacal aperture: Between two pelvic fins, on the tail region an elongated median groove or cloacal aperture is found. It leads into a small chamber, the cloaca, which is the common exit for digestive and urinogenital system. 5. Abdominal pores: Within either lateral edge of cloaca, the abdominal pores are situated on elevated papillae.
The abdominal cavity opens to the exterior through these abdominal pores. 6. Caudal pits: At the base of caudal fin the tail bears two shallow depressions, one dorsal and one ventral, known as caudal pits, which are characteristics of the genus scoliodon. 7. Lateral line and pores: A faint lateral line runs along either lateral side of the body. It marks the position of an underlying sensory lateral line canal system which opens outside at intervals through minute pores. 8. Ampullary pores: On the head and snout several minute ampullary pores of the ampullae of lorenzini open at dorsal surface.
They secrete mucus when pressed.