COELENTERATA (hollow gut)
Coelenterata are simple animals. This phylum includes jellyfish, corals and rarer forms commonly known as sea pens and sea whips. Another name for the phylum Coelenterata is the phylum Cnidaria (named for the stinging cells that these animals used to capture prey). Their bodies are radially symmetrical. All have two layers of cells, outer (ectoderm) and inner (endoderm), that form the body wall. These cellular layers are separated by a non-cellular substance called mesogloea.
Coelenterates are different from sponges in that they: 1) have stinging tentacles; 2) have muscles that enable them to change shape; and 3) secrete digestive juices from glands in the endoderm. The mouth is surrounded by tentacles that push food into a sack-like stomach cavity called the coelenteron (whence the name of the phylum). In the coelenteron, food is digested and distributed; waste is expelled through the mouth.
Coelenterates lack respiratory and circulatory organs and blood. They have a simple nervous system in the body wall. Coelenterates reproduce sexually, asexually by budding, and by alternation of these two processes. Most are marine animals and live in all temperatures and depth ranges of the oceans. Some characteristics of past environments can be determined if coral reefs are found in rock, because almost all reef-forming corals live in tropical. shallow, well -illuminated, low-nutrient (non-polluted) water. Coelenterates began evolving in the Precambrian (pre-543 million years ago) and are living now.
Honeycomb coral (genus Favosites) - This fossil gets its common name from its resemblance to a honeycomb. Each six-sided cylinder in the fossil is the protective limy shell of a single animal - the animal, called a polyp, has long since decayed. The polyp and its protective covering are part of a coral head formed by a coral colony. The horizontal layers that extend between the walls of the skeleton are named tabulae. These are floors that the polyp secreted as it grew and moved higher off the sea bottom. Favosites and other Paleozoic corals with tabulae are assigned to the extinct coral order Tabulata. All tabulate corals were colonial. Colonial means that all of the animals that compose a coral head are genetically identical, and are clones produced by asexual budding. This honeycomb coral existed during the Devonian (400 million years ago).
Horn coral (order Rugosa) - The order Rugosa gets its name from the wrinkles on the
fossils' tough exterior covering. These horizontal wrinkles are the results of growth spurts
(perhaps seasonal). The common name "horn coral " used for solitary rugose corals comes from their resemblance to a cow horn. Septa can be seen at the top or calyx of the coral. Septa are partitions that divided the hollow stomach cavity (coelenteron) into compartments. The septa radiate from the center of the calyx. Horn corals lived as individual coralites, or formed colonial coral heads composed of a number of coralites. They, like the honeycomb coral, appeared in the Late Cambrian (ca. 500 million years ago) and went extinct just before the Age of Dinosaurs (ca. 220 million years ago).
Conularida (class) - Conularids are an
extinct group of marine animals. Recent work
(in 2002) in the State Museum shows their earliest
appearance in the Middle Cambrian (ca.
505 million years ago). Conularids are known
from thin-walled, cone-like external shells.
These shells have a square cross-section.
Shallow depressions along the four edges of
the shell and along the midline of each face of
the shell are present. Additional recent State
Museum work also shows that conularids
were attached to the sea floor at their tips. The
square cross section of conularids and sedentary
(fixed in one place) life mode are known
in the larval stages of some modem jellyfish.
For these reasons, many paleontologists regard
conularids as a group within the phylum
Coelenterata. Conularids seem to have had a
similar mode of life as horn corals, but preferred
muddier, poorly illuminated sea floors.
Conularids disappeared in the Carboniferous.
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