My young readers interested in geology are probably familiar with the curious _Conularia_, or cone in cone, which has been found in Australia sixteen inches in length, and has always been regarded a puzzle. It has been suggested that this is a gigantic fossil Pteropod. The little needle-like Tentaculites, from the Silurian and Devonian rocks, are also allies. Some of the most remarkable mollusks are found among the sea-slugs, so called from their resemblance to the slugs of the garden. I have found them on the weed floating in the Gulf Stream, so resembling the latter in almost every particular that it was difficult to determine that they were not a part of the weed itself. _Scyllaea pelagica_ is such a form; helpless, yet finding protection in its mimicry of the surroundings. Equally as remarkable is _Dendronotus_,[36] the bushy sea-slug whose gills resemble the branches of weed in a remarkable manner. This curious sea creature is quite common on the seaweeds of our New-England beaches. In the Mediterranean and Pacific is found the most unique of the group, the _Phyllirhoë bucephala_ (Plate V., Fig. 2), which differs from many so entirely that it would seem to belong elsewhere. Like the other forms, it is pelagic, often being seen swimming along, resembling a fish, with its compressed body, and vertical, fan-like tail, and with long feelers or tentacles ahead. It is transparent and shelless in the adult stage, possesses no foot or _branchiæ_, evidently breathing through the body-walls or general surface. To add to its curious features, the _Phyllirhoë_ is brilliantly phosphorescent; light being emitted from certain spots, shown in the engraving, rendering the tissues transparent and luminous. Examination has shown that the light proceeds from certain globular nucleated cells, which appear to be the terminations of nerves. The _Phyllirhoë_ thrives well in the aquarium, and has been studied and observed in the famous aquarium at Naples. When it is touched or is swimming, the light seems to diffuse the entire surface, so that it presents a striking contrast against the dark water; and undoubtedly this gift is a fatal one, attracting the attention of many a fish to the dainty morsel seemingly outlined in fire. Giglioli refers to the luminosity of an undescribed Heteropod, the axis of whose body gave out a reddish light whenever the animal was excited. According to C. W. Peach, the young of _Eolis_ are phosphorescent. Such instances where the animal is particularly defenceless are amusing refutations of the theories of naturalists who see in the light a warning.