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. 11a). _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. 11b), 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. [Illustration: Fig. 11. Morphology and principal parts of corals. (a) Solitary or “horn” coral. (b) Colonial or compound coral.] a Columella Septum Corallite b Calyx Septum Corallum The class Anthozoa has been divided into several subclasses, but only one, the Zoantharia, is of paleontological importance. Subclass Zoantharia.— Most corals and all sea anemones belong to this subclass. Zoantharians are either colonial or solitary and, because most of them possess a hard preservable exoskeleton, they are the most important group of anthozoans geologically. The various orders of the subclass Zoantharia are discussed below. Order Rugosa.— These are corals in which the septa are arranged in cycles of four. Both solitary and colonial forms occur, and they are found only in rocks of Paleozoic age. Rugose corals are abundant in many of the Paleozoic formations of Texas, and two of the more typical forms (_Lophophyllidium_ and _Caninia_) are illustrated in Plate 15. Members of this order have been placed in the subclass Tetracoralla of older classifications. Order Scleractinia.—