Corals comprise another important branch of the Coelenterates. During the Cambrian period there were corallike sponges and possibly simple corals, but from the early Ordovician to the present true corals have been common, especially in the clearer, warmer seas. Their carbonate of lime skeletons have accumulated to help build up great limestone formations representing almost every geologic age from early Paleozoic time to the present. Paleozoic corals were in general notably different from those of later time. There were three main types including the compound "honeycomb" and "chain" types, and the solitary or compound "cup" type. They all had four, or multiples of four, radiating partitions; were rarely branched; and were generally large, some individual cup corals ranging in length from half an inch to a foot or more. Modern corals (beginning with the Mesozoic) have six or eight partitions; are nearly all profusely branched; and are mostly tiny individuals. Echinoderms are all marine animals, including the so-called "starfishes," which are not really fishes. They have body cavity, with digestive canal, low order nervous system, and a water circulatory system. Most of them have radially segmented shells or skeletons. The oldest fossil forms are found in Cambrian strata, these being very simple or primitive types, with a bladderlike head set on the end of a segmented stem, both head and stem having been supported by carbonate of lime. Such forms lived only to middle Paleozoic time. Ordovician strata contain representatives of all the main types of Echinoderms in well-fossilized forms. [Illustration: Fig. 51.--Fossil Echinoderms or so-called "starfishes": a, simple type known as the "stone lily" with head, stem, and roots intact from Silurian strata; b and c, irregular and regular higher type Echinoderms called "sea urchins" from Cretaceous strata.]