Final answer:
The wet form of Feline Infectious Peritonitis causes fluid accumulation in the abdomen or chest, known as ascites or pleural effusion, which can be life-threatening for cats.
Step-by-step explanation:
The form of Feline Infectious Peritonitis (FIP) that causes fluid to accumulate in the abdomen or chest cavity is known as the effusive form or the wet form of the disease. When cats are affected by this form of FIP, the inflammation leads to the leakage of fluid into the abdominal or thoracic cavities, resulting in ascites (abdominal fluid retention) or pleural effusion (fluid retention in the chest cavity). This condition can be severe and life-threatening, as it can interfere with the normal function of internal organs and impede breathing when fluid accumulates in the chest cavity.