The numbers after the arrows represent the number of the questions that should be considered next.
Explanation:
In the dichotomous key, the characteristics of the fish that will be considered must be carefully observed in order to develop a two-option system. Taking into account the fish features, the dilemmas generated will carry the number 1 (1a or 1b). Each option will present a description of the organism, and at the end of that choice there will be another number (arrow plus number), which belongs to the next feature described or questions asked about the organism to take into account to identify the taxonomic group. Depending on the option chosen, that question will lead to another question to be answered.
The correct answer is the second option. Animals have two major types: vertebrates and invertebrates. Vertebrates are animals that have backbone while the invertebrates do not have one. Examples for vertebrates are birds and mammals while the latter are insects and squid.
