This phylum also includes animals such as the jellyfish and sea anemone and the organisms that make up coral reefs. The nerve fibres are primitive because they do not form axons or dendrites, moreover the nerve fibres form actual contacts with fibres of other nerve cells. 31.17). The tentacle holding the captured animal contracts and bends over the mouth, the other tentacles also bend and help to transfer the food into the mouth where it is engulfed by movements of the mouth and hypostome; peristaltic contractions of the body wall force it into the enteron. Our mission is to provide an online platform to help students to share notes in Biology.
The endodermal cells get arranged into a layer beneath the ectodermal layer and, thus, a new cavity called coelenteron or gastro vascular cavity appears. Hydra prefers mild temperature which suits best for its life activities, say from 20 to 25° C. Any increase or decrease from these levels in temperature is avoided by Hydra.
31.22) develops as a simple evagination of the body wall. The messages radiate in all directions from the point of stimulation but there is no coordination because the messages do not evoke responses equally in all the effectors. Welcome to BiologyDiscussion!
Gonads may also be present on its body. Hydra (Science: organism) genus of freshwater coelenterates (cnidarians). Nitrogenous wastes are largely in the form of ammonia, which also diffuses through the general body surface.
The hypostome bears an aperture at its apex called mouth which opens into the gastro vascular cavity or enteron.
At the proximal end of the body, it may bear lateral projections called buds in various stages of development.
The ectoderm of basal disc is glandular which helps in fixing the Hydra with the substratum; its central part can produce gas bubble which helps in floating.
Each head may be again split in a similar manner. While standing erect, the body first extends and then bends and fixes the tentacles to the substratum by means of glutinant nematocysts. They are small, solitary and only exist in the polyp form, which is a radially symmetrical cylinder that is attached to the substratum at one end and has a mouth surrounded by tentacles at the other.
Some endoderm cells form pseudopodia and engulf the smaller partly digested particles into food vacuoles. Behaviour of Hydra depends on its physiological state, the response of a well-fed Hydra to stimuli is slow and sluggish, but a hungry Hydra will respond vigorously to the same stimuli. Hydra always shows negative response to injurious chemicals but exhibits positive response to food.
The two nerve nets are joined to each other and to the sensory cells of both ectoderm and endoderm, they are also joined to the epitheliomuscular cell. If Hydra is attached near the bottom, then the body is held upright, but at average depth it is horizontal with the hypostome lower than the foot. Search Britannica The algae are passed from one generation of Hydra to the next through the eggs.
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