The vast numbers of _medusæ_, and their importance as light-givers, may be realized from the remarks of Giglioli, who states that their light was seen from the “Magenta” over an area of forty-four degrees of latitude, and for nearly thirty consecutive days. During the day they sank into the greater depths, at night rising to the surface, and appearing like moderator lamps. With their long groups of tentacles trailing behind as they pulsate through the ocean waters, they readily suggest the title, “Meteors of the Sea.” With few exceptions, the sea-jellies are light-givers. The giant _Cyanea_,--one of which was measured by Mrs. Agassiz, and found to be nearly six feet in diameter, and to have tentacles over one hundred feet in length--emits a pale, greenish light; and, if the entire mass is luminous, it must present a wondrous appearance as it moves through the water, like a gigantic meteor. As large as this giant is, weighing many hundred pounds, it is produced from a delicate little creature which would hardly be noticed by the casual observer. One of the commonest forms along the New-England coast is a diminutive jelly,[4] seemingly blown in glass by some skilful worker. As it moves gracefully along, it emits a light of a deep aurelian blue, vast numbers imparting a metallic glitter to the water. On some calm night, about a rocky point where the current flows silently along, myriads of these wondrous forms may be seen passing in review. Peering down into the depths from our boat, we may see a pretty, shapely jelly-fish, called _Zygodactyla_, a golden _ignis fatuus_ of the ocean waters; the _Melicertus_, another of the same family, surrounded by a golden radiance; and a stately _Rhizostoma_, which Giglioli observed in fresh or brackish water in Batavia, emitting a fixed, bluish light; while _Zina_, _Coryne_, _Eucope_ and _Clytia_, and a host of other exceedingly pretty sea-jellies, add to the glories of the scene. The delicate _Thaumantius_ (Plate III., Fig. 3) and Oceanea are resplendent light-givers. The latter, according to Ehrenberg, being “surrounded by a shining crown,” while _Pelagia_ illumines the deep sea by its mystic rays. Although we have established a rule to refer the most of the technical names, with the more scientific matter, to the Appendix notes, we are yet inclined to retain in the text, occasionally, some names which are especially attractive. Thousands of marine animals have no other name but the generic ones given them by discoverers; but in many instances they are pretty, and there is no reason why they should not be used, as they must become the common name of the object, as well as its technical one. Other known light-givers are recorded in the Appendix,[5]--all forms of the greatest delicacy and beauty.