This class contains a group of one-celled animals which may secrete an exoskeleton (external protective covering) of chitin, silica, or calcium carbonate. Included in this class are foraminiferans (commonly called forams) and radiolarians. Order Foraminifera.— Members of this order secrete tiny chambered shells which are very useful microfossils. The forams are predominantly marine organisms and have shells composed of chitin, silica, or calcium carbonate. In addition, some forms construct a shell of sand grains or some other material which is cemented together by a sticky substance that is secreted by the animal. Forams are very abundant in the rocks of Texas and particularly so in rocks of Mesozoic and Cenozoic age. The most numerous and easily observed Paleozoic foraminiferans are the fusulinids (fig. 9a), and their small spindle-shaped remains are very abundant in many of the Pennsylvanian limestones of north-central and Trans-Pecos Texas. Some typical Texas forams are illustrated in figure 9. Order Radiolaria.— The radiolarians (fig. 10) have delicate spine-covered shells composed of silica, and their remains are very abundant in certain recent marine sediments. They may also be found as fossils and have been reported from Devonian and Permian rocks in Trans-Pecos Texas, and probable radiolarians have been reported from still younger beds. [Illustration: Fig. 9. Typical Texas Foraminifera (all greatly enlarged). (a) _Fusulina_ (Pennsylvanian). (b) _Robulus._ (c) _Globigerina._ (d) _Frondicularia._ (b-d, Cretaceous).] [Illustration: Fig. 10. Typical radiolarians (greatly enlarged). (a) _Actinomma_ (Recent). (b) _Porodiscus_ (Eocene).] 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.