Arthropoda overview
The Arthropoda phylum is the largest phylum to this day and there are over 1 quintillion individuals alive that belong to this phylum. This phylum contains all insects, which is one of the reasons that the number of animals in this phylum is so big. Arthropods are the easiest identified of all animals, as they have several things in common that no other animals have. Arthropods also have a very complex internal system. They complete digestive, excretory, circulatory, and respiratory systems. Arthropods can walk, crawl, climb, fly, swim, and almost every possible type of moving. They are found on every continent (even Antarctica), and in every ocean.
Insects
Insects begin their respiratory system when air enters through openings in the exoskeleton called spiracles. After passing through the spiracle, the air enters through the tracheal trunk, and then diffuses through a network of tracheal tubes that reaches to almost every part of the body. At the end of each tracheal tube is a small cell called a tracheole that allows for the exchange of gasses from atmospheric air and into the cell. In some parts of the body where there is no taenidia (which is cuticle reinforcement), air sacs are formed which can store some air for reserve purposes.
Centipedes
Centipedes have similar respiratory systems to insects. Air is brought into the body through spiracles located on each segment of the body. The same process occurs such as in insects. The air is brought through a network of tracheal tubes until they are exchanged into the cell at the tracheole. The air is then exhaled out through the spiracles.
Crayfish
Crayfish are a type of fish that live in freshwater, and are related to the lobster family, except they are a little smaller. The way this fish breaths is pretty odd. Their gills are on the outside of the body but they are attached to the legs of the crayfish as well. Therefore, the more it walks, the more it breathes. However, if it wants to, it can walk on land and breathe, because it has a small pocket within itself where it stores water when it needs it. This is a very unique respiratory system, and no other create has something of this sort, making this a very distinct animal.