^A. flavidula.^ Size eight to ten inches in diameter; disk gelatinous, transparent bluish-white, broad and comparatively flat, with a fringe of short tentacles of even length around the margin; margin broken by eight notches, in each one of which is a club-like organ (/tentaculocyst/) containing calcareous spots or eyes, which are hidden by lappets or hood-like coverings; groups of nerve-cells also lie in the marginal notches; radial canals branched; manubrium very short, with square mouth, which is surrounded by delicate membranes, or oral arms, each arm being a folded membrane tapering to a point. The edges of the membranes are covered with lasso- or stinging-cells. Four egg-sacs, or gonads, are conspicuous in horseshoe shape around the center of the disk. The gonads are pink in the males, and yellow in the females. These medusæ swim in shoals, and are common everywhere in summer. They may be said to be annual animals, for they make their appearance regularly as free-swimming medusæ in the latter part of April, when they may be seen in immense numbers near the surface when the water is smooth and the sky clear. At this time they are about an inch in diameter. They grow rapidly, and by the end of June have attained their full size. At [pg138] the end of July they are fully developed, and begin to discharge their eggs, which go into the folds around the mouth and remain there until they attain the planula stage. After the spawning period the medusæ, reduced in strength, are unable to resist the storms of the autumn, and many of them are cast ashore; many others, in a more or less wasted condition, float near the surface, but the body is less transparent, its tissues are thickened, its tentacles gone, and general dissolution has commenced. In this condition the medusæ are frequently capsized by the air which accumulates in the empty egg-cavities, and, floating helplessly on the surface, are attacked and destroyed by swarms of small crustaceans; thus their cycle of life is terminated. It has been suggested that the destruction of the mothers, by being cast upon the beaches in the autumnal gales, is a provision to set free the planulæ in a position favorable to their existence; for when liberated they fasten upon the rocks and seaweeds of the shore, where, during the winter months, they develop into strobilæ, which in turn free their saucer-like disks early in April. [Illustration: Strobila of /Aurelia flavidula/.] GENUS ^Cyanea^ [Illustration: /Cyanea arctica/, greatly reduced in size.]