. Elements of comparative anatomy. Anatomy, Comparative. 116 COMPARATIVE ANATOMY. of the umbrella is continued into the wall of the stomach in the Discophora, the stomach is not very sharply marked off from the rest of the gastric system. Its wall is always continued into arm-like appendages, which, as a rule, project into folded membranes (oral arms); the mouth is placed between these. Division of these arms leads to further modifications, which give rise to greatly ramified appendages. In this case numerous grooves, which gradually unite, lead to the mouth, in correspondence with the form of
. Elements of comparative anatomy. Anatomy, Comparative. 116 COMPARATIVE ANATOMY. of the umbrella is continued into the wall of the stomach in the Discophora, the stomach is not very sharply marked off from the rest of the gastric system. Its wall is always continued into arm-like appendages, which, as a rule, project into folded membranes (oral arms); the mouth is placed between these. Division of these arms leads to further modifications, which give rise to greatly ramified appendages. In this case numerous grooves, which gradually unite, lead to the mouth, in correspondence with the form of the arms. In the Khizostomidas the mouth remains open during an early period of development only, and afterwards becomes closed by the gradual union of the " arms," which limit it, and in which the grooves form branched canals, which open at the ends of the ramifications of the arms by numerous fine pores (polystomia). In the Lucernariae the structural conditions of the gastro- vascular system closely resemble those of the Medusas. A stomachal tube, projecting from the concave surface of the umbrella, and pro- duced into four angles, leads into a wide space, which is continued into four pouches, and may be elongated into four canals, which pass into the stalk. The four pouches correspond to widened radial canals, and are, as in the Medusas, connected with one another at the edge of the umbrella, and so form a circular canal. In others this character is modified in such a way that the stomach is continued into the body, in a tubular form; and at its end, which projects into the stalk, gives rise to radial canals, which whilst becoming enlarged run outwards towards the margin of the disc. The gastrovascular system in the larva? of the Discophora and in Scyphostoma is very similar in character. § 91. The gastric system of the Anthozoa ex- tends by means of an oesophagus from the centre of the tentacle- bearing surface of the body into the interior, where it opens into
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