BIO 002: INVERTEBRATE BIOLOGY PHYLUM COELENTERATA

PHYLUM COELENTERATA (CNIDERIA)

The coelenterates include more than 9000 living species. They are all aquatic, mostly marine but also with a few fresh water forms. They are often found abundantly in warm temperature or subtropical waters. These animals are carnivores, they actively do not move from place to place, rather they lie in wait and capture their prey (e.g. fishes, crustaceans) with the tentacles that ring their mouth. Examples include jelly fishes, sea anemones, corals, hydroids etc.

Characteristic Features of Coelenterates

 Coelenterates are multicellular organisms  They have tissue-grade of organization  All members of this phylum are aquatic animals  They are radially symmetrical or the symmetry of a wheel  The body wall is diploblastic. It is made up of two layers of cells, namely the ectoderm and the endoderm with a non–cellular layer called mesogloea in between  They have no coelom, hence coelenterates are acoelomate animals  Two individual forms of coelenterate exist: polyps and medusa  Respiratory, excretory and circulatory system are absent  Life history has alternation of generations or metagenesis.

Forms of Coelenterate

Coelenterates may have two basic forms which are:

 Polyp  Medusa

Polyps are cylindrical and are usually found attached to a firm substrate. They may be solitary or colonial. In a polyp, the mouth faces away from the substrate on which the animal is growing and is therefore often facing upwards. The polyp is the sedentary, benthic form

In contrast, most Medusa free-floating and are often umbrella shaped with their mouth usually pointing downwards and the tentacles hanging down around them.

Polyp and Medusa form of Coelenterates

Classification of Coelenterata

Depending mainly upon whether polyp or medusa form is the dominant form in the life cycle, coelenterates are divided into four classes: hydroza (hydroids), scyphoza (jellyfish), Cuboza (box jellyfish), and Anthozoa (sea anemones and corals).

Class hydroza

1. Hydrozoa are solitary and fresh water or mostly colonial and marine, sessile and free- swimming forms. 2. Body wall consists of an outer ectoderm and an inner endoderm separated by a non–cellular gelatinous mesogloea 3. Many hydrozoa exhibit alternation of generation 4. Reproductive products of sex cells are usually ectodermal in origin and discharged externally 5. Gastrovascular cavity without stomodaeum 6. They exhibit polymorphism. 7. Examples include: Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia Companularia, Millepora, Stylaster, Geryonia, Physalia, Porpita, Velella, Pericolpa, Periphylla, Aurelia, Cynaea, Rhizostoma or Pilema Cassiopeia, etc.

Class Scyphozoa 1. They are exclusively marine. 2. Medusae are large, bell or umbrella-shaped and without true velum. They are free swimming or attached by an aboral stalk. 3. Gonads are endodermal and the sex cells are discharged into the stomach 4. Class scyphozoa is divided into five orders, namely Stauromedusae, Cubomedusae, Coronatae, Semaeostomeae and Rhizostomeae 5. Polypoid generation is absent 6. Mesogloea is usually cellular 7. Examples include Lucernaria, Haliclytus, and Aurelia etc.

Class cubozoa

1. They are solitary medusoid forms 2. They have reduced polyp stage 3. Medusa is box shaped 4. All are marine animals 5. A tentacle or group of tentacles is found at each corner of the box 6. Example include Carybdea, box jellyfish

Class Anthozoa

1) All are polyps 2) They are solitary or colonial 3) Gonads are gastrodermal 4) All are marine 5) Example include sea anemone

HYDRA

Kingdom: Animalia

Phylum: Cnidaria

Class: Hydrozoa

Order: Anthoathecata

Family: Hydridae

Genus: Hydra Linnaeus, 1758[1]

Hydra is a genus of small fresh water organism usually found in temperate or tropical regions. It belongs to the phylum Cnidaria and class Hydrozoa

It has a tubular, radially symmetric body up to 10 mm (0.39 in) long but could attain a length of 20mm or more when its body fully extended.

The closed end of hydra is known as the foot or the basal disc refered to as the aboral end.

Usually have small hydras known as buds attached to them when food supply is in abundance.

They have rounded projections on their body which are sexual organs- testes and ovaries.

Hydra has two main body layers, which makes it "diploblastic". The layers are separated by mesoglea, a gel-like substance. The outer layer is the epidermis, and the inner layer is called the gastrodermis, because it lines the stomach.

Hydra does not possess a recognizable brain or true muscles, rather nervous system of Hydra is a nerve net. Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles

Respiration and excretion occur by diffusion everywhere through the epidermis

Motion and Locomotion

Hydra are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting. They have two distinct methods for moving – 'looping' and 'somersaulting'. They do this by bending over and attaching themselves to the substrate with the mouth and tentacles and then relocate the foot, which provides the usual attachment, this process is called looping. In somersaulting, the body then bends over and makes a new place of attachment with the foot. By this process of "looping" or "somersaulting", a Hydra can move several inches (c. 100 mm) in a day. Hydra may also move by amoeboid motion of their bases or by detaching from the substrate and floating away in the current.

Reproduction

When food is plentiful, many Hydra reproduce asexually by producing buds in the body wall, which grow to be miniature adults and break away when they are mature. When a hydra is well fed, a new bud can form every two days.[5] When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some Hydra. Swellings in the body wall develop into either an ovary or testes. The testes release free-swimming gametes into the water, and these can fertilize the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating, and, as the adult dies (due to starvation and/or cold), these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into nymph Hydra.

Feeding

Hydra are carnivorous mainly feed on aquatic invertebrates such as Daphnia and Cyclops. When feeding, Hydra extend their body to maximum length and then slowly extend their extraordinarily extensible tentacles which could be four to five times the length of the body Once fully extended, the tentacles are slowly maneuvered around waiting for contact with a suitable prey animal. Upon contact, nematocysts on the tentacle fire into the prey, and the tentacle itself coils around the prey. Within 30 seconds, most of the remaining tentacles will have already joined in the attack to subdue the struggling prey. Within two minutes, the tentacles will have surrounded the prey and moved it into the opened mouth aperture. Within ten minutes, the prey will have been engulfed within the body cavity, and digestion will have started. Hydra are able to stretch their body wall considerably in order to digest prey more than twice their size. After two or three days, the indigestible remains of the prey will be discharged through the mouth aperture via contractions.

Adaptive features of hydra

Foots secretes a sticky substance for Anchorage and Lcoomotion

Interstitial cells which is chief agent in regeneration, repairs, budding and reconstructing tissues in growth

Nematoblast helps in defence

Hydra usually remains attached by its basal disc or foot to objects under water

The movement of hydra are for: the capture of the prey; response to stimuli; locomotion

Swimming is facilitated by the wave-like movement of the tentacles Hydra is a Carnivorous animal

Egestion of wastes is through the mouth

Gaseous exchange is made by almost direct diffusion through the cell membrane to the surrounding water