Is a ant a crustacean insect arachnid or millipipede?

Arthropod

Although all 4 are types of animals they are quite different.

• An Arthropod is an invertebrate with an exoskeleton rather than an internal skeleton (and hence not a vertebrate since that would require having a backbone or the equivalent), a segmented body, and jointed paired appendages.
• A mollusk is also an invertebrate but has a mantle with a significant cavity used for breathing and excretion, a radula, a minutely toothed, chitinous ribbon kind of like a tounge, which is typically used for scraping or cutting food before the food enters the esophagus (except in the case of bivalves like clams and oysters, and a nervous system with two pairs of main nerve cords (one mostly serving the internal organs and the other the "foot") organized around a number of paired ganglia. In many species of mollusks the epidermis of the mantle secretes calcium carbonate and conchiolin which create a shell for the mollusk.
• The term "worm" applies to a disparate group of animals with long cylindrical tube-like bodies and no legs. Most, but not all, worms are invertebrates.
• Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them. They lack any nervous, digestive, or circulatory system. Most sponges rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Obviously they live in the water since they rely on this water flow.