Cnidaria structure
Musculo-epithelial cells in Cnidarians
The major cell types of the Cnidaria are the musculo epithelial cells, nerve cells, and nematoblasts which produce nematocysts. The ‘body’ is composed mainly of musculo-epithelial cells. These cells, found only in the cnidarians, are a means of producing an epithelium combined with a contractile system. Each cell consists of two parts, epithelial and muscular, linked by cytoplasm.
The epithelial portion lies externally and forms the outer covering of the animal in the ectoderm as well as the lining of the enteron in the endoderm. The muscular portion contains muscle fibres that lie next to the mesogloea along the base of the cell. The muscle fibers are arranged longitudinally in the ectoderm and transversely in the endoderm so that they work antagonistically.
The musculo-epithelial cells of the endoderm may be modified into glandular and digestive cells bearing flagella. Glandular cells produce enzymes that are secreted into the enteron and break food material into small particles. Digestive cells engulf these particles which then undergo further digestion in the food vacuoles within the cells.
Nerve cells in Cnidarians
Nerve cells have been positively identified only in medusae and anthozoans. The nervous system in cnidarians takes the form of a nerve net composed of a large number of multipolar nerve cells, that is,
nerve cells with many dendrites or connecting links. Because any stimulus must cross the numerous synapses or gaps between cells, the transmission of the impulse is very slow. In some species, transmission is accelerated by the development of conduction paths, regions where the nerve net is composed of bipolar cells orientated to form a pathway where synaptic resistance is reduced.
Mesogloea in Cnidarians
The mesogloca is a secretion lying between the two cell layers of the body wall. In medusae the mesogloea constitutes the bulk of the animal. Thought to be produced mainly by the cells of the ectoderm, this secretion is composed of a protein similar to collagen. Its function is important because it forms a base upon which the muscle system can operate and therefore acts in a skeletal capacity, preventing excessive deformation of the body.
Feeding habits in Cnidarians
Cnidarians are carnivorous, feeding on living animals such as small fishes and their eggs, small crustaceans, molluscs, worms, and other cnidarians. They capture their prey and convey it to the mouth by means of their tentacles which possess nemato cysts.
Nematocysts are characteristic of the phylum, but some molluscs and some flatworms which prey on cnidarians are able to collect them in their own tissues and use them. They are double-walled capsules containing a coiled and sometimes barbed thread which is ejected when the cnidocil or trigger comes in contact with a foreign body.
As the coiled thread is ejected from the capsule it turns inside out, and its fine point pierces the prey, injecting a paralyzing toxin. A toxin from the nematocysts of a large jellyfish or Portuguese Man o’ War can produce a kind of ‘allergic’ response in the form of rashes and weals in human beings, and the poison of some siphonophores is lethal.
Other types of nematocyst have adhesive powers and some coil around the victim. Nematocysts are not controlled by the nervous system but are discharged by chemical and mechanical stimulation. Large numbers are released and are continually replaced, as once released they are expended.
The tentacles then deliver the prey to the mouth and enteron where it is broken down into small particles for ingestion by the cells lining the enteron. Soluble food is distributed, possibly by diffusion, to cells of the body wall of the polyp, and via the canal system in medusae.