The below mentioned article provides a study note on the Locomotion in Hydra.

Introduction to Hydra:

Hydra represents one of the smallest soli­tary polyp amongst the cnidarians; most of them inhabit fresh water bodies except Proto-hydra which is marine. Hydra normally remains attached to and hanging downwards from the substratum in water, by their basal disc. The body stalk and tentacles can extend, contract, or bend to one side or the other.

In hydras the gastro-dermal fibres in most part of the body are so poorly developed that the movement almost entirely depend on the contraction of longitudinal epidermal fibers. Besides, fluid within the gastro-vascular cavity plays an important role in locomotion as hydraulic skeleton.

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By taking the water through the mouth, a relaxed Hydra may stretch out to a length of 20 mm, whereas con­traction of the epidermal fibres can reduce it to a mere 0-5 mm. For capturing the prey or to change the position, hydras can detach and shift locations by different types of movement.

Organs helping in Locomotion: Basal Disc:

The aboral end of the tubular body of Hydra is named as basal disc or foot (Fig. 1.32D). It is used as a structure for attachment to the substratum. In this region, the epithelia muscular cells are modified into tall cells with granular cytoplasm.

These cells produce a sticky secretion that helps the animal to adhere to the substratum under the water. Sometimes, these cells also produce gas bub­ble which enables the animal to float in water.

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Tentacles:

In hydras, the mouth is situated at the summit of a conical elevation called the hypostome. The base of the hypos- tome is surrounded by tentacles (Fig. 1.31), the number of which vary from four to eleven in different species.

In the tentacles, cnidoblasts are numerous and are arranged in groups to form small surface tubercle or battery. In these cnidoblasts two types of nematocysts are found – larger (9 µm) oval- shaped glutinant streptoline and smaller (7 µm) glutinant stereoline. Both types of nematocysts help in locomotion by produ­cing sticky secretion.

Epitheliomuscular Cells:

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The myoep­ithelial cells are the most common type of cells found in the epidermis of hydras. The cells are columnar in shape, with the base resting upon mesoglea and the distal end forming the epidermal surface.

These cells possess two or more basal extensions, each containing a contractile myofibril. The ends of successive extensions are connected to each other, forming a cylindrical longitudi­nal, contractile layer (Fig. 2.18). The contrac­tion of this layer shortens the body column and the tentacles.