. Elementary text-book of zoology. AURELIA. 129 The mouth passes by a short oesophagus into a gastric cavity which is produced into four pockets in the inter-radii. Internal Each pocket contains on its oral wall a horse- shoe-shaped gonad, and near the middle a row of gastric filaments which assist in digestion. The gastric cavity is continued outwards towards the edge of the disc by numerous vascular canals. The eight primary branched canals are the four per-radial and the four inter- radial. Between these there are the eight secondary . un- branched canals or ad-radials. All the canals open
. Elementary text-book of zoology. AURELIA. 129 The mouth passes by a short oesophagus into a gastric cavity which is produced into four pockets in the inter-radii. Internal Each pocket contains on its oral wall a horse- shoe-shaped gonad, and near the middle a row of gastric filaments which assist in digestion. The gastric cavity is continued outwards towards the edge of the disc by numerous vascular canals. The eight primary branched canals are the four per-radial and the four inter- radial. Between these there are the eight secondary . un- branched canals or ad-radials. All the canals open into a ring- canal round the edge of the disc. The gastric cavity and the canals are ciliated. They are derived from the coelenteron, as' in Obelia. In the inter-radii, immediately below the gonads, are four sub-genital pits, each opening on the oral surface by a pore. The mesogloea between the two layers is a thickened jelly which in this case contains scattered cell-elements. Fig. 61.—Median Longitudinal Section through the Inter- radial Plane of Aurelia. (Diagrammatic.) Gastric Filaments. Sub-genital Pit. J Stomach, Gonad. Inter-radial Canal. Ring-canal. Lappet. Or,-il Tentacle. Mouth. There is no nerve-ring, but there is a diffuse nerve-plexus concentrated round the sense-organs or tentaculocysts. These latter are complex and appear to unite the senses of sight, hearing and smell in different part*. Development. — Aurelia is dicecious and the sexual elements are discharged by the mouth. A free-swimming planula larva (Chapter V.) settles down on rocks or weeds and forms the hydra-tuba, a minute hydra-like individual. It is a two-layered -sac, with a mouth at the M. TO. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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