Animals with a nervous system; often with stinging cells. The body has a single cavity with only one opening (no separate anus). Free-swimming coelenterates (medusoids) have marginal tentacles; ben-thic forms have tentacles round mouth on the upper surface, and usually have radial symmetry. Includes many soft-bodied forms (e.g. jellyfish), but also the corals, which are important as fossils.
class PROTOMEDUSAE (Late Precambrian to Ordovician)
Jellyfish-shaped animals with radial lobes. Probably free-swimming, marine.
class HYDROZOA (Late Precambrian to Present)
A varied group of soft- and hard-bodied marine and freshwater coelenterates. It includes Ediacaria, a medusoid from the Late Precambrian. Many are colonial, including the coral-like order Milleporina (Cretaceous to Present) which occur on coral reefs and, unusually for hydrozoans, have a calcareous skeleton.
class SCYPHOZOA (Cambrian to Present)
Mainly soft-bodied, marine jellyfish, which are seldom preserved as fossils. This class also includes the order Conulariida (Cambrian to Triassic) which have the body protected by a cone or a four-sided pyramid of chitinophosphatic material; like the jellyfish, conulariids have a 4-rayed symmetry, and are thought to have been entirely marine and free-swimming or attached.
class ANTHOZOA (Late Precambrian to Present)
Benthic coelenterates, with the central cavity radially partitioned by fleshy mesenteries. The mouth is surrounded by retractable tentacles. Entirely marine.
ubclass OCTOCORALLIA (Late Precambrian to Present)
Colonial anthozoans with eight tentacles and eight mesenteries in each individual polyp. The skeleton is usually of calcareous spicules, sometimes it is a rigid calcareous framework; these latter forms common as fossils from the Cretaceous to the Present. Includes the order Pennatulacea (the sea pens), which have unbranched individuals anchored by a calcareous stalk, or possibly (in the case of the Late Precambrian Charnia and Rangea) by a circular calcareous disc. Most octocorallia are benthic shallow marine animals, but some have been recovered from the sea floor at very great depths.
ubclass ZOANTHARIA (Cambrian to Present)
Includes sea anemones and corals. Only the orders containing corals are listed here.
order Tabulata (Cambrian to Permian)
Colonial corals in which the individual polyps had their tubes partitioned by horizontal plates (tabulae), while the vertical radiating plates (septa), seen in most other coral groups, were rudimentary or absent. Some of this extinct group may, in fact, not be coelenterates but sponges.
order Rugosa (Ordovician to Permian)
Solitary or colonial corals, usually with alternating longer and shorter septa, showing bilateral or radial symmetry. Tabulae and other internal plates were usually present. Colonial corals were restricted to shallow water, but the solitary corals occurred over a wider depth range. The name is derived from rugae (broad ribs) which occur on the external walls of many of these corals.
order Heterocorallia (Carboniferous)
A small group of very elongate solitary corals.
order Scleractinia (Triassic to Present)
Solitary or colonial corals with radial septa in successive cycles (starting with 6 in the first cycle). Abundant small plates or rods between the septa. Scleractinias are the most abundant corals alive today. Some are dependent for their existence on the presence of large numbers of single-celled algae (dinoflagellates or zooxanthel-lae) in their tissues. These (hermatypic) corals have a maximum depth range of about 90m, and include most of the modern reef builders. Those corals not dependent on the light-requiring algae (ahermatypic corals) can live in all depths of water, but most prefer depths of less than about 500m in areas with slow sedimentation rates.
PHYLUM ANNELIDA and other phyla of worm-like animals
Occasional impressions of soft-bodied segmented annelids occur from the Late Precambrian (Ediacara fauna) to the present day. More frequently some hard protein jaws of certain annelids (mostly polychaetes) may be preserved fossil; these are known as scoleco-donts and range from the Cambrian. Most scolecodonts are marine, like the majority of present day polychaetes. Worm burrows, trails and the calcareous tubes are more abundant; many of these cannot be assigned to a phylum, but some traces are characteristic of particular groups (like the lining of terebellid tubes by cemented shell fragments). The calcareous tubes of serpulids can also be attributed to a particular group of polychaete worms. Some poly-chaetes which crawl on the sea floor leave characteristic trails; others, like the sabeflids, form mucus-lined burrows.
The paucity of fossils worms contrasts with their abundance in modern marine environments; they include free-swimming, crawling, sessile and burrowing animals. They also have a variety of feeding habits; in addition to the filter-feeding majority, they include carnivores and scavengers.
The sipunculid worms (Cambrian to Present) are assigned to a separate phylum from the annelids. Their bodies are not segmented, and most of them narrow anteriorly; the anterior extension is retractable and bears tentacles. Sipunculids are mostly infaunal deposit feeders.
Was this article helpful?