Q.Name the phylum, among those studied in this chapter, whose body cavity is present but is not completely lined by mesoderm.

A true (eu)coelom is a body cavity completely lined by mesoderm on both the body-wall side and the gut side, connected by mesenteries; a pseudocoelom is lined by mesoderm only on the outer (body-wall) side, with the gut wall left without a mesodermal covering — this fuller mesodermal lining gives the true coelom real structural and functional advantages.

Advantages of a true coelom over a pseudocoelom:

• Proper organ suspension: because the true coelom is fully lined by mesoderm and has mesenteries (double-layered mesodermal sheets), internal organs can be firmly suspended and held in position within the body cavity, rather than lying loosely.
• Better organisation of organ systems: the complete mesodermal lining allows organ systems (digestive, circulatory, reproductive, excretory) to develop with proper walls, muscular layers, and blood supply, permitting more complex and specialised organs than is possible in a pseudocoelomate body plan.
• Cushioning and hydrostatic function: the coelomic fluid within a true coelom cushions internal organs against shocks and can also act as a hydrostatic skeleton, aiding movement (as in earthworms). …