[Phylum] Porifera
These are sponges and are the simplest of the many-celled animals. Living sponges secrete a skeleton which may be composed of [chitin], [silica], or calcium carbonate. These substances are commonly found in the form of spicules—tiny hard parts that are used to help support the soft tissues of the animal. These spicules take on a [variety] of shapes ([Pl. 14]) and are occasionally found as microfossils in some marine sediments.
Although sponges are not particularly common fossils, their remains occur in some parts of the State. Sponges have been collected from [Paleozoic] and [Mesozoic] formations of north and Trans-Pecos Texas, and their spicules have been reported from well cuttings.
[Phylum] Coelenterata
The coelenterates are multicelled animals which, though more complex than the sponges, are rather primitive animals. The living animal is characterized by a sac-like body cavity, a definite mouth, and tentacles which bear stinging cells. Some forms, for example, the jellyfishes, have an umbrella-shaped body and are single free-moving organisms. Others, like the [colonial] corals, are composed of many individuals living together in a colony.
Most zoologists and paleontologists recognize three classes of coelenterates: (1) the Hydrozoa, containing the small animals known as hydroids, (2) the Scyphozoa, which includes the jellyfish, and (3) the Anthozoa, which includes the corals and sea anemones. Because of their extreme fragility and lack of hard parts, hydrozoans and scyphozoans are not commonly found as fossils. They do, however, have a long geologic history and may be preserved when unusual conditions of fossilization occur. The anthozoans, especially the corals, are by far the most important class geologically, and these forms have left a very good paleontological record.
CLASS ANTHOZOA.—
This class is composed of a group of exclusively marine organisms and includes the corals and sea anemones. The [coral] animal, or [polyp], secretes a cup-shaped [calcareous] (limy) [exoskeleton]. This skeleton, called a [corallite], is usually divided by radial partitions called septa. The polyp lives in the [calyx], which is the central bowl-shaped depression in the top of the corallite ([fig. 11]a).
[Solitary] corals form an individual [corallite] for each [polyp], and because of their shape these may be given such names as “horn corals” (Lophophyllidium, [Pl. 15]) or “button corals” (Micrabacia, [Pl. 16]). [Colonial] or compound corals ([Pl. 15]) live together in colonies, which are formed of many individual skeletons attached to each other ([fig. 11]b), and the compound mass of [coral] skeletons formed in this manner is called a [corallum]. [Fossil] corals commonly occur in many marine limestones and in places constitute a large portion of the [rock].
