TASK DEFINITION: Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
PROBLEM: Fact 1: Echinoderms use pheromones to communicate with each other. 
Fact 2: Ascothoracican barnacles attach to echinoderms or corals.

SOLUTION: corals use pheromones to communicate with each other.

PROBLEM: Fact 1: Gametes then unite in fertilization and form a diploid zygote. 
Fact 2: Zygotes are diploid and undergo cell division to become an embryo.

SOLUTION: Gametes then unite in fertilization and eventually become an embryo.

PROBLEM: Fact 1: Arthropods have a life cycle with sexual reproduction. 
Fact 2: Scorpions are vulnerable because they have the lowest rate of reproduction among arthropods.

SOLUTION:
scorpions have a low rate of reproduction.