The light of _Salpæ_ observed by Giglioli was confined to the so-called nucleus, but was not constant; indeed, some were luminous and some were not. This was particularly evident in the month of September, when the exploring-ship “Magenta” passed through a bed of these little creatures, fifteen miles in extent. Some observed in the South Atlantic had the nucleus tinged with a brilliant red light. Very similar to _Salpa_ is _Doliolum_, which seems to burn with a vivid green light scattered over the entire body, and is one of the emeralds of the sea. In the very lowest order (_Copelatæ_) of the Tunicates, we find an interesting, indeed remarkable, light-giver, the _Appendicularia_ (Plate XVI., Fig. 3). It resembles a tadpole with quite a long tail, retaining in its adult life features that only characterize the _larvæ_ of others of the group. Professor Agassiz has noticed two specimens on the New-England coast, and they are very common in both tropical and temperate waters of various regions. Some of the species are veritable house-builders, forming a gelatinous protection covering called a test. This habitation, if so we may term it, is formed or secreted with considerable rapidity, and is quite an elaborate affair; having two front chambers and a middle one large enough for the tail to move with ease. Curious to relate, this transparent residence is, according to Filhol, only used a few hours, being then deserted and another formed; so that its life would seem to be spent in making houses and deserting them. The light of certain _Appendiculariæ_ is almost as remarkable as that of the _Pyrosomæ_, in the variety of its coloring; one, according to Giglioli, appearing first red, then blue, and finally green. The seat of the luminosity, which appears in intense flashes, was the central axis of the tail, or caudal appendage. Between Montevideo and Batavia in the South Atlantic, this naturalist observed many of these little creatures, nearly all of which showed these tri-colored favors; and in the Indian Ocean some were seen emitting white, blue, and green lights, marking them as among the most striking of all the light-givers. Charles William Peach, an English naturalist, has observed the tadpole form of _Cynthia_ to emit light. _Cynthia pyriformis_ resembles a peach in form, size, and even bloom; its tests having rich reddish tints. It is a familiar form in deep water from Cape Cod to Greenland and across to Scandinavia. It is one of the most common objects on our New-England beaches after storms. The heavy seas throw it up from its hiding-places. To the student or interested visitor it is a beautiful object. CHAPTER XII. FINNY LIGHT-BEARERS.