In this article we will discuss about the interrelationships between the different classes of phylum Cnidaria. Class Hydrozoa represents the most primitive forms amongst the Cnidarians. The lowest hydrozoan form is exhibited by Hydra and the hydrula stage of some Hydrozoa. The relationship between Hydrozoa and Scyphozoa is not very clear. In spite of this, it can be suggested that [...]
In this article we will discuss about the habit, habitat and life history of Obelia. Habit and Habitat of Obelia: Obelia is a typical representative of colonial hydrozoa (Fig. 12.14). It is exclusively marine. It exists in two principal forms, the polyp and the medusa. The polyp or hydroid form represents the asexual phase of its life-history and the medusoid [...]
The following points highlight the two main types of polymorphic variabilities in Siphonophora. 1. Modifications of the Polyp: A. Gastrozooids or Trophozooids or Nutritive Zooids: Characters: 1. It is like a typical polyp (Fig. 12.34A,B). 2. Each gastrozooid is a cylindrical and funnel-shaped body with a large mouth. 3. Presence of a well-formed coelenteron. 4. A single long contractile and [...]