Nematodes and arthropods are grouped together in the Ecdysozoa; which shared characteristic best explains this classification?

Introduction / Context:
This question tests your understanding of animal phylogeny, specifically the grouping of nematodes and arthropods within the Ecdysozoa. Modern classification is based on shared derived characteristics rather than simple body shape. The key feature uniting these phyla is their mode of growth through moulting, which differentiates them from other linages such as lophotrochozoans and deuterostomes.

Given Data / Assumptions:
- The groups mentioned are nematodes, commonly called roundworms, and arthropods, which include insects, crustaceans and spiders.- The options refer to feeding strategy, larval type, embryonic development, moulting and body cavity type.- The question asks which characteristic they share that explains their placement in the Ecdysozoa.- We assume a basic familiarity with major animal phyla and their traits.

Concept / Approach:
Ecdysozoa is a clade defined by the process of ecdysis, which is periodic shedding of a tough external covering. Nematodes have a flexible but tough cuticle that must be shed to allow growth, while arthropods possess a chitinous exoskeleton that they moult at intervals. This shared trait is a synapomorphy for the group. Feeding strategies vary widely among these animals, and their larvae are not uniformly ciliated. Embryonic development patterns and body cavity types also differ between nematodes and arthropods, so those cannot be the key unifying feature. Therefore, the correct option is the one that highlights growth by shedding an external cuticle or exoskeleton.

Step-by-Step Solution:
Step 1: Recall that Ecdysozoa are animals that grow by moulting an external cuticle in a process called ecdysis.Step 2: Think of nematodes and remember that they have a non cellular cuticle that is shed multiple times as they grow.Step 3: Think of arthropods and remember that they possess a rigid exoskeleton made of chitin, which must be replaced periodically.Step 4: Compare this shared moulting process with the options given.Step 5: Choose option D, which explicitly states growth by shedding an external cuticle or exoskeleton.

Verification / Alternative check:
A phylogenetic tree in most modern zoology texts labels nematodes and arthropods as part of Ecdysozoa based on ecdysis.You will also find detailed life cycle diagrams showing moulting stages in both groups, reinforcing the idea that shedding the cuticle is a central feature.Since none of the other traits listed are shared uniquely by both groups, this confirms the chosen answer.

Why Other Options Are Wrong:
Option A is wrong because not all nematodes and arthropods are suspension feeders; many are predators, parasites or scavengers.Option B is wrong because ciliated larvae are more characteristic of some lophotrochozoans and are not a defining shared trait of nematodes and arthropods.Option C is wrong because the pattern of blastopore fate differs between these groups and does not serve as the main basis for grouping them together.Option E is wrong because nematodes are pseudocoelomate and arthropods are coelomate; neither group is universally acoelomate.

Common Pitfalls:
A common error is to focus on ecological or behavioural similarities rather than on the structural and developmental traits used in modern phylogeny.Students may also misremember the body cavity type of nematodes and assume that similar size or shape is enough to group animals together.Keeping the term ecdysis associated with Ecdysozoa helps you quickly recall that moulting is the unifying feature.

Final Answer:
Nematodes and arthropods are grouped together because both grow by periodically shedding an external cuticle or exoskeleton.