The following points highlight the top two phases of obelia. The phases are: 1. Hydroid or Polyp Phase 2. Medusoid Phase.

Obelia: Phase # 1. Hydroid or Polyp Phase:

Hydroid stage of Obelia is in a colonial form. It is a branched filament like structure and remains attached with the substratum. This colony is polymorphic, i.e., number of individuals or zooids are present which are morphologically as well as functionally different from each other.

The polyp is com­posed of following structures:

(a) Hydrorhiza and Hydrocaulus:

ADVERTISEMENTS:

Obelia colony is constituted of two portions, the horizontal portion is called the Hydro­rhiza and the vertical portion bearing the zooids is named as the Hydrocaulus (Fig. 1.36).

(b) Gastro-zooid or Trophozooid or Nutritive Zooid:

Most of the zooids present in the hydroid stage of Obelia are the Gastro-zooid (Fig. 1.36 and 1.37A). They are specially designated to perform nutritive function and feed the whole colony. Each Gastro-zooid has a short tube-like body having at its distal end a conical projection called the hypostome or manubrium.

The mouth is situated at the terminal end of the manubrium. Surrounding the manubrium there is a circlet of about twenty four solid tentacles.

ADVERTISEMENTS:

(c) Blastostyle or Gonozooid or Reproductive Zooid:

These particular types of zooids are few in number in comparison to the number of gastro-zooids. Each blastostyle has a long cylindrical body without mouth and tentacles (Fig. 1.37B). The coelenteron is greatly reduced. It is enclosed by a transpa­rent covering called gonotheca. The lateral wall of the body gives off small lateral buds called the medusa-buds.

Obelia: Phase # 2. Medusoid Phase:

Medusa develops as hollow offshoot of the blastostyle. When fully-formed, it assumes the appearance of an umbrella with a convex surface by which the medusa was attached with the blastostyle (Fig. 1.37C). This convex side is called the ex-umbrella and the concave side of the umbrella is known as the sub-umbrella.

From the centre of the sub- umbrellar surface emerges a hanging tube called the manubrium bearing a square mouth at its terminal end (Fig. 1.37D). The edge of the umbrella gives rise to a very short circular shelf called the velum. At the junc­tion of the exumbrellar side and velum there is a circlet of tentacles.

ADVERTISEMENTS:

The number of tentacles is sixteen in a newly-formed stage but the number may increase