Biology · Animal Kingdom · NEET
Aschelminthes are PSEUDOCOELOMATES. This is the single most-tested fact about them. A true coelom is a body cavity fully lined by mesoderm. In roundworms the mesoderm is not a full lining — it is present only as scattered pouches between the ectoderm (body wall) and endoderm (gut). A cavity like this, not fully lined by mesoderm, is called a pseudocoelom ("false coelom"). Do not confuse them with Platyhelminthes (flatworms), which are ACOELOMATE — they have no body cavity at all.
Because of WHERE the mesoderm is. In a true coelomate (like annelids), mesoderm lines both the body wall side AND the gut side of the cavity. In a pseudocoelomate, mesoderm is present towards the body wall but is missing as a continuous lining around the alimentary canal. So the cavity is only partly lined. NEET has directly asked this: an animal with mesoderm towards the body wall but none around the gut is a pseudocoelomate (NEET 2025).
They are TRIPLOBLASTIC. They have all three germ layers: ectoderm, mesoderm, and endoderm. Only Porifera and Coelenterata (Cnidaria) and Ctenophora are diploblastic (two layers). From Platyhelminthes onwards, all animals — including Aschelminthes — are triploblastic. So "pseudocoelomate" does NOT mean "only two germ layers". They still have mesoderm; it is just not arranged as a full cavity lining.
COMPLETE. The alimentary canal of Aschelminthes runs from mouth to anus, with a well-developed muscular pharynx. This is a jump up from Platyhelminthes, which have an INCOMPLETE gut (only one opening). Remember: roundworms are the first phylum in the NEET sequence with a complete, tube-like digestive system and organ-system level of organisation.
Yes. Aschelminthes are DIOECIOUS — sexes are separate, meaning males and females are distinct individuals. Often the females are longer than the males. Fertilisation is internal, and development may be direct (young resemble adults) or indirect. Contrast this with Platyhelminthes (flatworms), which are mostly hermaphrodite (monoecious). This male-vs-female difference is a favourite NEET comparison point.
Common NEET examples: Ascaris (the round worm — intestinal parasite), Wuchereria (causes filariasis / elephantiasis), Ancylostoma (hookworm). Wuchereria is important because filariasis/elephantiasis is matched to phylum Aschelminthes in human-health PYQs. Note that they may be free-living (aquatic or terrestrial) OR parasitic in plants and animals — not all roundworms are parasites.
While trying to find out the characteristic of a newly found animal, a researcher did the histology of adult animal and observed a cavity with presence of mesodermal tissue towards the body wall but no mesodermal tissue was observed towards the alimentary canal. What could be the possible coelom of that animal?
Consider the following statements: A. Annelids are true coelomates. B. Poriferans are pseudocoelomates. C. Aschelminthes are acoelomates. D. Platyhelminthes are pseudocoelomates. Choose the correct answer.
Bilaterally symmetrical and acoelomate animals are exemplified by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The body of Aschelminthes is circular in cross-section, so they are called roundworms. If you slice one across, the cut face looks like a round ring — unlike the flat-body flatworms (Platyhelminthes).
Organ-system level of body organisation. This is higher than the tissue level of Coelenterata and the organ level of Platyhelminthes.
A true coelom is fully lined by mesoderm on all sides; a pseudocoelom is only partly lined, with mesoderm present as scattered pouches and absent around the gut.
Yes. An excretory tube removes body wastes from the body cavity and releases them through the excretory pore. This is a defined, tube-based excretory route.
Phylum Annelida — the segmented worms. Annelids are the first TRUE coelomates in the sequence and show metameric segmentation, a big step up from the pseudocoelom of roundworms.
