Why is a scorpion not a crustacean?

Outwardly, scorpions are very similar to small crayfish, but belong to the class arachnids, because:
– on their cephalothorax, like spiders, eight walking legs;
– live on land and therefore breathe not with gills, like crayfish, but with the help of pulmonary sacs and trachea, like spiders;
– like spiders, scorpions have two types of digestion – external and internal: under the influence of poison, the victim is digested, and then the scorpion sucks out the internal contents.