Master Notes on Cnidarians (With Diagram)

Master Notes on Cnidarians:- 1. Protohydra 2. Halistemma 3. Physalia 4. Porpita 5. Velella 6. Diphyes.

Cnidarian # 1. Protohydra:

Protohydra (Fig. 12.29A) is a peculiar hydra of 3 mm in length. It lacks the usual tentacles. It is available along the shores of Great Britain and northern part of Europe. Protohydra feeds on nematodes and small crustaceans and propagates usually by longitudinal and transverse fission. It is dioecious and lacks a medusoid stage.

Cnidarian # 2. Halistemma:

Halistemma (Fig. 12.28) rep­resents a typical form of Siphonophore colony.

Some peculiar features are:

(i) Largest nematocyst amongst Cnidarains is present in Halistemma. The length of the nematocyst is about 1-12 mm and the length of the dis­charged tube is about several millimetres long.

(ii) The Halistmma colony consists of a long, slender, floating stem and the upper end of the stem bears a small, expanded, gas-filled float or pneumatophore, serves to propel the colony through the water.

(iii) Two alternate rows of the swimming bells or nectocalyces are closely pressed together to assume bilateral shape.

(iv) Below the last nectocalyx, a cluster of individuals of the colony includ­ing different zooids or polyps hang from the main stem, called cormidia. Each cormidium contains a gastro­zooid, dactylozooid, phyllozooid (bract as hydrophyllium) and gonozooids.

(v) When newly formed, the different zooids of the colony are located on one side, but subsequently the stem on which the zooids are borne be­comes twisted and appears to be irregular.

Cnidarian # 3. Physalia:

Another remarkable Siphonophore is Physalia, commonly known as Portuguese man-of-war (Fig. 12.28).

When touched the long trailing tentacles liberate toxic substances to produce severe pain to other organisms.

The noted features are:

(i) The size may reach with a float up to 10 to 30 cm in length.

(ii) Swimming bells and tentilla are absent.

(iii) The gaseous content of the air-sac con­tains 1.5% Argon, 85 to 91% Nitrogen, and 7.5 to 13.5% Oxygen.

(iv) A remarkable instance of commensalism is present between Physalia and the fish, Nomena. This small fish swims among the tentacles of Physalia.

(v) The float is an elongated bladder like sac with two pointed ends and ranges from 3 to 12 cm in length.

(vi) From the under-surface of pneuma­tophore of some pelagic siphonophores (e.g., Portuguese man-of-war, Physalia and in others) hang the complex colony of zooids or polyps in clusters, called cormidia.

Each cormidium contains leaf-like phyllozooids, and other zooids are gastrozooid, dactylozooids and gonozooids. The phyllozooids are used for protection and respiration, gastrozooids for feeding, dactylozooids for stinging and gonozooids for repro­duction.

(vii) The female reproductive zooids be­come detached from the colony as free medusae, but the male ones remain attached.

Cnidarian # 4. Porpita:

Porpita (Fig. 12.28) represents the most modified form amongst the Chondrophora.

Some peculiar features of Porpita are:

(i) The float forms a round discoid body with air-chambers. Each of these cham­bers communicates to the outside by many pores.

(ii) The centre of the colony is occupied by a single large gastrozooid.

(iii) The edge is provided with simple hol­low tentacle-like bodies—the dactylo­zooids (tentaculozooids).

(iv) The gonozooids or blastostyles are like gastrozooids and may possess mouth but tentacle is absent.

(v) The nectocalyces or swimming bells are absent.

(vi) Beneath the float, there is a mass (so- called liver) permeated with endodermal canals which are regarded to be excretory in function. The walls of these canals are filled with guanine crystals.

Cnidarian # 5. Velella:

Velella (Fig. 12.28) is a closely allied genus of Porpita and it also exhibits a form amongst Chondrophora. Both of them belong to the same group and under a single suborder.

Some special features of Velella are:

(i) The float (Pneumatophore) is like that of Porpita but it possesses an oblique shell on the upper side and it has rhomboidal form.

(ii) The gastrozooid, dactylozooids and gonozooids are all like those of Porpita, but tentilla is absent.

(iii)Velella contains 88% water, 5.6% salts, 3% chitin and the rest is covered by other organic substances. Traces of car­bohydrate and fat are also recorded.

(iv) Just like Porpita, the so-called liver performing excretory function is present.

Cnidarian # 6. Diphyes:

Diphyes represents a peculiar Siphonophore colony, where the float is lack­ing. The proximal end of the colony is occu­pied by swimming bells.

The peculiar features are:

(i) The swimming bells are two in number and are similar.

(ii) The bells are followed by groups of zooids. Each group comprises of a polyp with tentacles, a medusoid form and a large bract encircling them.