Answer: receptors, external
Step-by-step explanation:
1. A sense organ is made up of receptors which are made up of cells or group of cells which can detect external stimuli like touch, light, sound and others.
2. Salmon reproduce by the mode of external fertilization. In this process both eggs and sperms are released in water. It occurs when females and males secreate their gametes in the same area of water. The females secreate eggs before males release sperms. The eggs get covered with sperms. The eggs get fertilized in water.