What animals do not have a double circulatory system?

Animals that do not have a double circulatory system primarily include most invertebrates and some lower vertebrates. For example, most arthropods, such as insects and crustaceans, have an open circulatory system, where blood is not confined to vessels. Additionally, some fish, like those in the class Agnatha (e.g., lampreys and hagfish), possess a single circulatory system, where blood flows in one circuit through the heart to the gills and then to the rest of the body.