Which is the largest animal phylum in terms of species count?

In which of the following animal groups is a closed circulatory system $NOT$ found?

In which phylum are compound eyes found in animals?

Which of the following is non-poisonous?

Open vascular system is found in

$A$ student has identified a triploblastic,coelomate,segmented animal as an arthropod. Which additional character should the student have verified before identifying that animal as an arthropod?