Spiders are multicellular invertebrate animals; to which zoological phylum are they classified according to standard animal taxonomy?

Introduction / Context:
Biological classification groups animals into hierarchical categories such as phylum, class and order. This question asks you to recall the phylum to which spiders belong. Knowing that spiders are arthropods reinforces important ideas about exoskeletons, jointed legs and segmented bodies, which are characteristic of one of the most successful animal groups on Earth.

Given Data / Assumptions:
- Spiders are familiar land dwelling invertebrates with eight legs.- The options list several major animal phyla: Mollusca, Annelida, Cnidaria, Arthropoda and one more for comparison.- You are expected to know basic features of these phyla from introductory zoology.- No calculation is needed, only conceptual knowledge of classification.

Concept / Approach:
Spiders have a hard external skeleton made of chitin, a segmented body, jointed appendages and an open circulatory system. These features are typical of the phylum Arthropoda. Within this phylum, spiders belong to the class Arachnida, which also includes scorpions and ticks. Other phyla listed in the options have very different body plans. Molluscs usually have soft bodies and many have shells. Annelids are segmented worms with no jointed legs. Cnidarians are radially symmetrical animals like jellyfish. Echinoderms such as starfish have a water vascular system and radial symmetry as adults. Therefore, the correct phylum for spiders is Arthropoda.

Step-by-Step Solution:
Step 1: Recall that arthropods are defined by jointed legs, segmented bodies and a chitinous exoskeleton.Step 2: Observe that spiders clearly have jointed legs and an exoskeleton, which fits the arthropod definition.Step 3: Recognise that spiders are also classed as arachnids, a group inside Arthropoda.Step 4: Compare this with the options and locate Arthropoda, described as joint legged invertebrates, in option D.Step 5: Choose option D as the correct phylum for spiders.

Verification / Alternative check:
Illustrated classification charts in textbooks show spiders alongside insects and crustaceans as members of phylum Arthropoda.Field guides to animals often list spiders in the arthropod section due to their shared body design.No authoritative source places spiders in the phyla Mollusca, Annelida or Cnidaria, which confirms that Arthropoda is the correct answer.

Why Other Options Are Wrong:
Option A is wrong because Mollusca includes snails, clams and squids that have soft bodies and often shells, not jointed legs with exoskeletons.Option B is wrong because Annelida includes earthworms and leeches, which have segmented bodies but lack jointed appendages and hard exoskeletons.Option C is wrong because Cnidaria covers jellyfish, corals and sea anemones, which have radial symmetry and stinging cells rather than segmented bodies with legs.Option E is wrong because Echinodermata includes starfish and sea urchins, which are marine animals with a unique water vascular system and pentaradial symmetry as adults.

Common Pitfalls:
Learners sometimes confuse arachnids with insects and may not realise that both are subgroups within Arthropoda, which can lead to uncertainty about the correct phylum.Another pitfall is to focus only on habitat, such as marine versus terrestrial, instead of core structural traits like jointed legs and exoskeletons.Remembering that arthropods are the joint legged invertebrates helps you confidently classify spiders in this phylum.

Final Answer:
Spiders belong to the phylum Arthropoda, the group of joint legged invertebrate animals.