Ctenophores and Cnidarians - first organism with real structure
The simplest organisms to possess such structures are the Ctenophores which include comb jellies, " sea gooseberries", " sea walnuts" and the " Venus' girdles" and the Cnidarians which are represented by the Anthozoa which are corals and sea anemones, Scyphozoa which are jellyfish, the Cubozoa represented by the box jellyfish (sea wasps) and the and Hydrozoa which includes the Hydroids, hydra-like animals. Chironex fleckeri is a highly venomous species of box jellyfish that inhabits Australian coast and is a very fast swimmer and has very sophisticated eyes.
One best known Ctenophores are the Comb jellies are voracious marine predators, feeding mostly on plankton. Ctenophores are mainly composed of inert mesoglea, which causes them to have a low rate of metabolism. Many species are bioluminescent. The name comb jelly comes from eight "comb rows" of fused cilia, called ctenes, which are arranged laterally along the sides of the animal and used primarily for locomotion. The ctenes of the ctenophores gives rise to a rainbow-like effect that is caused by scattering of light due to the beating of cilia, not because of bioluminescence. The ctenophores are hermaphroditic, and some species can reproduce asexually. Most ctenophores have two long tentacles, but some lack tentacles completely. The tentacles have adhesive structures called colloblasts, or lasso cells. These cells burst open when prey comes in contact with the tentacle. Sticky threads released from each of the colloblasts will then capture the food. Some species have their entire body surface covered with sticky mucus that captures prey. There are about 100 modern species of these marine animals. One of the most familiar genera of ctenophore is Mnemiopsis. Due to their soft and fragile bodies, the fossil record for comb jellies is poor. One possible ctenophore is known from the Middle Cambrian period.
The Cnidarians are more common and have bodies clearly divided into two cellular layers, each layer one-cell thick. The outer layer of cells is the ectoderm whereas the inner layer is the endoderm. The individual cells of the ectoderm are specialized for various functions such as protection, secretion, defence and cell replacement whereas the endoderm is specialized for digestion, absorption and assimilation of food. The stinging cells (Cnidocytes) of the ectoderm are highly specialized and contain coiled threads inside. When food or an enemy comes near, the cell discharges the thread which is armed with spines like a miniature harpoon and often loaded with poison. These cells are often concentrated at the ends of tentacles. Cnidarians reproduce by releasing eggs and sperm into the sea. The fertilized egg first develops into a free swimming creature that is quite different from its parents. It eventually settles down at the bottom of the sea and develops into a tiny flower-like organism called a polyp which filter-feed with the aid of tiny-beating cilia. Eventually, the polyp bud in a different way and produce miniature medusae which detach themselves and once again become free-swimming. True jellyfish spend most of their time as free-floating medusae with only the minimum period fixed to the rocks as solitary polyp, whereas sea anemones do the reverse with most of their life spent attached to rock as solitary polyp. Yet other coelenterates exist as colonies of polyps which have given-up a sessile life and have become free-floating e.g. Portuguese Man O'War (Physalia).