Animal Kingdom - Aschelminthes

Body cavity organisation is a key basis of classification in NCERT Class 11, Chapter 4 (Animal Kingdom), and on this basis Aschelminthes, the roundworms such as Ascaris and Wuchereria, possess a cavity that is not lined on all sides by mesoderm; this is termed a pseudocoelom or false coelom, making such animals pseudocoelomates. This represents an organisational advance over the acoelomate flatworms. Hence the false coelom or pseudocoelom is the correct feature. A true coelom lined entirely by mesoderm is incorrect, because that condition, the eucoelomate state, is found in higher phyla such as Annelida, Arthropoda and Chordata, not in roundworms. The complete absence of any body cavity describes the acoelomate Platyhelminthes, the more primitive condition that roundworms improve upon. A haemocoel filled with blood is a feature of arthropods and molluscs, not of aschelminthes. Roundworms additionally show organ-system level organisation, bilateral symmetry, and often pronounced sexual dimorphism, with separate sexes. A consistency check confirms the reasoning: since flatworms lack a cavity and roundworms have one that is only partly mesoderm-lined, the pseudocoelom logically marks the intermediate evolutionary step described between the acoelomate and the truly coelomate grades.