Question

Obelia is
A. Sedentary
B. Attached but capable of locomotion
C. Motile
D. Free floating

Answer 1

Hint: Obelia is a genus belonging to class Hydrozoa and phylum Cnidaria. It consists of very small marine animals that form colonies. The colonies are found attached to different substrates like surfaces of sea weeds, wooden piles, rocks, and shells of molluscs.

Complete answer:
• The genus Obelia is cosmopolitan showing wide distribution. It is found in shallow water within oceans.
• Obelia is commonly known as sea fur as it is delicate and appears as a light brown or white colored plant like fur.
• It shows colonial form with each colony having horizontal thread like root which is known as hydrorhiza. Hydrorhiza is attached to the substratum.
• Hydrorhiza gives rise to a vertical branching stem called hydrocaulus. Both hydrorhiza and hydrocaulus are hollow tubes with the same thickness. Obelia thus has two types of filaments – horizontal hydrorhiza and vertical hydrocaulus. Each hydrocaulus shows an alternate branching system which terminates into polyps. All the polyps are collectively known as zooids. Zooids are of three types – nutritive zooids, sexual zooids and budding zooids. Thus, Obelia colony shows trimorphism.
• The polyps in Obelia show some type of locomotion. They can show some movements under the influence of water current. The polyps have longitudinal and circular muscles in their body wall that also allow them to contract and extend. However, the obelia colony is incapable of showing locomotion as it is firmly attached to the substratum.

Hence, the correct answer is option (A).

Note: Obelia is a small marine colonial form that occurs in shallow ocean water attached to a substratum like rock, shell etc. It consists of polyps that can show some movements. However, the attachment of the obelia colony to substratum makes it sedentary/sessile/immobile.