Which type of animals do not possess a bladder?

The correct understanding lies in recognizing that certain animals have evolutionary adaptations which influence their physical structures and organ functions. Among those mentioned, poultry, which primarily refers to birds, do not possess a bladder in the way mammals do. Birds have a unique urinary system where the kidneys excrete waste materials, which then pass through ducts directly to the cloaca, a common exit for both urinary and digestive waste. This adaptation is significant in minimizing weight for flight and helps them maintain a waterproof system with less water retention.

In contrast, mammals generally develop a bladder as a storage organ for urine, allowing them to control urination and maintain homeostasis. Fish, on the other hand, often have a swim bladder, which aids in buoyancy but does not serve the same function as a urinary bladder. Reptiles do possess a type of urinary bladder, but it varies by species and serves to store urine before excretion. Thus, the absence of a true bladder in poultry highlights their unique evolutionary path and functional adaptations compared to other animal classes.