Cnidarians and Ctenophores: next step in evolution!

Phylum CNIDARIA (jelly fish, anemones, corals)

Cnidarians and Ctenophores represent the next step in evolution: radial symmetry. After the primitive cell aggregation of sponge, these are the first two Phyla of animals presenting this new structure.

A part of radial symmetry, they present a body with two layers of tissue, tentacles bearing stinging cells called cnidocytes and a digestive cavity with a single opening (mouth and anus at the same time! BLEACH!)

It is an important phylum presenting more than 9000 species, living in the marine environment. Jelly fish, sea anemones, corals and sea pens belong to this Phylum.

In relation to sponge, cnidarians present also a primitive nervous system. There is no a proper brain but several nerve cells that respond to tactile and chemical stimulus. Cnidaria are anyway quite simple in construction. Neither respiratory nor secretor systems are present.

A cross section of the body wall shows two layers: outside an ectoderm with the cnidocytes and inside an endoderm. In between a jelly like material called mesoglea is present. The mesoglea could be very thin, like in anemones, or very thick, like in medusas.

Cnidocytes are present in all cnidaria and are located on the tentacles. They contain threads called nematocysts that can be ejected by tactile or chemical stimulation for defence or prey capture.

Cnidaria body shape is in two basic types: polyp or medusa. Polyp is sessile and presents the mouth end up. Medusa is planktonic and has the mouth and the tentacles hang down. One is the upside-down form of the other.

In some classes, cnidaria alternate a planktonic medusa stage, during their larval phase, with a sessile polyp stage, during their adult phase. Other cnidaria have only one form for all their life, either medusa or polyp. Besides, many species are individual animals while a great number too are colonial, presenting morphological and functional specialization among the individuals.

Cnidarians are divided in 4 Classes, based on their structure.

Anthozoa: (sea anemone and coral). It is the largest of the 4 classes, presenting about 6000 species. It occurs only in the polyp form and could be both individual (anemones) and colonial (corals). The animals present numerous septa through the all body, which increase the surface area of the digestive tissue. Most of them belong to the group of the Hexacorals and present 6 or a multiple of 6 tentacles.

Schyphozoa: (jelly fish). It is present with about 200 species of large planktonic cnidaria. Medusa is the dominant form with big and solitary individuals. Several species have also a sessile polyp stage. Many species are highly poisonous even for human been.

Hydrozoa: (hydroids). It present about 3000 species of small colonies, where prevails a colonial polyp stage which alternates with a solitary medusa stage. In this class the life cycle is very complex and generation alternation is the rule. A particular group is represented by the Siphonophore, which presents complex colonial pelagic medusae.

Cubozoa: (cubomedusae). It presents few species of cubic jelly fish with 4 long tentacles used for the prey capture. Polyp stage is much reduced. They are highly poisonous and they can cause even death in case of multiple stinging. They are present in shallow tropical seas.

Phylum CTENOPHORA (sea gooseberry, sea walnut)

Like Cnidaria, Ctenophora have single digestive cavity, two body layers with mesoglea in between and tentacles with specialized cell for prey capture.

It is a small phylum with only about 90 marine species; most of them are pelagic and component of the zooplankton. The majority of them are spherical and measure few cm of diameter. Few species have a flat body up to 1 m long, like the belt of Venus.

Differently from Cnidaria, Ctenophores present a peculiar locomotion system with cilia. Cilia are particular structure organized in combs (ctena in ancient greek) arranged in row, which overlap each other. There are 8 rows of ciliary combs from the top to the bottom of the animal. Thanks to the uniform beating of cilia, ctenophores can swim.

Moreover, ctenophores present on the top of the tentacles, specialized cells called colloblasts. These cells contain a coiled thread that is discharged under stimulation, which capture and immobilize preys from the zooplankton.

Even if ctenophores are transparent, light reflection on the cilia gives the impression that they have rainbow colours on their comb rows.

… it continues! …

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