Identifying the Phylum of Asterias and Ophiura

The question asks about the classification of two organisms, Asterias and Ophiura, at the phylum level. To answer this, we need to recall the characteristics of different animal phyla and the common examples belonging to them.

Let's look at the organisms mentioned:

• Asterias: This is the scientific name for a common starfish.
• Ophiura: This is the scientific name for a brittle star.

Both starfish and brittle stars are marine invertebrates known for their radial symmetry (usually five-fold in adults) and the presence of a water vascular system used for locomotion, feeding, and respiration. These characteristics are defining features of the Phylum Echinodermata.

Phylum Echinodermata Explained

Echinoderms ("spiny-skinned") are exclusively marine animals. Key features include:

• Adults typically exhibit pentaradial symmetry (though larvae are bilaterally symmetrical).
• Presence of an endoskeleton made of calcareous ossicles.
• A unique water vascular system with tube feet.
• Lack of a head or brain.
• Examples include starfish (Asterias), brittle stars (Ophiura), sea urchins, sand dollars, and sea cucumbers.

Examining Other Options

Let's consider why the other options are incorrect:

• Phylum - Arthropoda: This phylum includes insects, spiders, crustaceans, etc. They have jointed legs and an exoskeleton made of chitin. Asterias and Ophiura do not fit this description.
• Phylum - Chordata: This phylum includes vertebrates (fishes, amphibians, reptiles, birds, mammals) and some invertebrates (like tunicates and lancelets). They possess a notochord at some stage of life. Asterias and Ophiura are not chordates.
• Phylum - Hemichordata: These are marine invertebrates, sometimes considered a link between echinoderms and chordates. Examples include acorn worms. They have a structure called a stomochord. Asterias and Ophiura are distinctly different from hemichordates.

Based on their characteristics and common examples, Asterias (starfish) and Ophiura (brittle star) clearly belong to the Phylum Echinodermata.