Q.Mention the animals that exhibited a "tube-within-a-tube" organisation for the first time? Name their body cavity.

The tube-within-a-tube organisation first appeared in the roundworms (Aschelminthes / Nematoda); their body cavity is a pseudocoelom.

Explanation:

• In the tube-within-a-tube plan, the body wall forms the outer tube and the alimentary canal forms the inner tube, with a complete digestive tract having a separate MOUTH and ANUS.
• This complete gut (with a distinct mouth and anus) appeared for the FIRST TIME in the animals of phylum Aschelminthes (Nemathelminthes / Nematoda) - the roundworms (e.g. Ascaris).
• (In the earlier phyla, Coelenterata and Platyhelminthes, the gut is incomplete with only a single opening.) …

Ascaris eggs are termed mammillated because their outer covering is studded with irregular, nipple-like (mammillary) projections, giving the shell surface a rough, bumpy texture.

Ascaris lumbricoides (the large roundworm, a common human intestinal parasite) produces fertilised eggs with a distinctive multilayered shell. From outside inward, the shell consists of:

1. An outer, coarsely mammillated (bumpy/warty) proteinaceous coat, giving the surface an irregular, nodulated appearance — this is the layer that gives the egg its name.
2. A middle, thick, transparent chitinous (glycoprotein) layer.
3. An inner, thin lipoidal (vitelline) layer.

The egg also often appears brownish/golden in colour because it absorbs bile pigments as it passes through the host's intestine.

…

Ascaris eggs are called 'mammillated eggs' because their outer shell bears numerous small, rounded (mammilla-like) projections that give it a knobby, uneven surface.

Justification: The fertilized egg of Ascaris lumbricoides is surrounded by several protective coverings. The outermost is an uneven, coarsely granular, albuminous (proteinaceous) coat secreted as the egg passes down the uterus. This layer is studded with irregular knob-like projections resembling small mammillae (nipple-like protuberances), giving the egg a characteristically bumpy, corrugated outline under the microscope — hence the name 'mammillated egg'.

…