Which one of the following phylum is characterised by absence of true coelom ?

Concept-

• The five-kingdom system was proposed by R.H. Whittaker.
• The kingdom defined by him was named- Monera, Protista, Fungi, Plantae, Animalia.
• Kingdom Animalia is divided into sub-groups which is known as Phylum.
• There are 11 phyla in the kingdom Animalia.
  • Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata.

• The presence or absence of a cavity between the body wall and the gut wall is very important in classification.
• The body cavity, which is lined by mesoderm is called coelom.
• Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.
• In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., aschelminthes.
• The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes (Nematoda).

Therefore Nematoda phylum is characterised by the absence of true coelom.

Additional InformationEchinodermata

• These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata.
• All are marine with the organ-system level of organization.
• The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
• They are triploblastic and coelomate animals.
• The digestive system is complete with a mouth on the lower (ventral) side and anus on the upper (dorsal) side.
• The most distinctive feature of echinoderms is the presence of a water vascular system which helps in the locomotion, capture, and transport of food and respiration. 

Mollusca

• This is the second-largest animal phylum.
• Molluscs are terrestrial or aquatic (marine or freshwater) having an organ-system level of organization.
• They are bilaterally symmetrical, triploblastic, and coelomate animals.
• The body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot, and visceral hump.
• A soft and spongy layer of skin forms a mantle over the visceral hump.
• The space between the hump and the mantle is called the mantle cavity in which feather-like gills are present.

Annelida

• They may be aquatic (marine and freshwater) or terrestrial; free-living, and sometimes parasitic.
• They exhibit an organ-system level of body organization and bilateral symmetry.
• They are triploblastic, metamerically segmented, and coelomate animals.
• Their body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida (Latin, annulus: little ring).
• They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present. Nephridia (sing. nephridium) helps in osmoregulation and excretion.