[19] PAGE 26. While investigations so far have failed to explain the physiology of the light, it has been found that in a perfect animal it is emitted from eight opaque cords, each of which passes from a little swelling at the base of a tentacle down each polyp into the covering of the branch. The cords are canals in the sarcode of the branch, connecting the hollow of each tentacle with the tubular cavities of the branchlets and stem. The microscope shows that the contents of the canals are a fluid and cells; the latter containing minute highly refracting globular particles of a fatty substance, which resists decomposition long after the death of the polyp itself. If these cords are ruptured, the luminosity of the entire mass is excited, and the fatty cell contents is luminous after its escape, and on foreign matter even after the death of the animal. Regarding the light, Duncan says, referring to Panceri’s experiments, “There is no sensible increase of temperature, and the tint of the monochromatic light is azure or greenish, but never red. In this beautiful instance of this remarkable vital luminousness there is evidently a photogenic structure and an elaborated organic material capable of producing light after removal from the animal. The sequence of illuminating the whole pen is slow,--far less than that of the movement of nerve-force. Yet the presence of the lowly organized nervous element indicates that the regulating of the light may relate to it as its function.” Perhaps the most magnificent of all the _Pennatulidæ_ is the tall _Umbellularia grænlandica_ (Plate XXI., Fig. 2), which consists of twelve huge polyps, each with eight fringed arms, terminating in a close cluster upon a stalk about four feet in height. This striking form was dredged by the “Challenger” expedition in water over two miles in depth, where the pressure is so great one can hardly realize it, and the temperature is just above freezing. Sir Wyville Thompson says, that, when this splendid animal was taken from the trawl, it emitted a light so brilliant that Capt. Maclear found it an easy matter to determine the character of the light by the spectroscope. It gave a very restrictedly continuous spectrum, sharply included between the lines _b_ and _d_. [20] PAGE 27. _Pavonia quadrangularis._ [21] PAGE 27. _Asteronyx loveni._ [22] PAGE 27. _Ophiacantha._ [23] PAGE 28. _Renilla reniformis._ [24] PAGE 28. _Virgularia_ is so named from its rod-like form; _vira_, a rod. _V. mirabilis_ is found off the English coast. [25] PAGE 30. _Ophiura_ and _Asterias_. These are genera of the sea-stars, or star-fishes long so called; the former so named on account of the resemblance to snakes in its arms. [26] PAGE 30. _Ophiothrix fragilis_, _Amphiura belli_, and _Ophiocantha spinulosa_. [27] PAGE 31. _Ophiocnida olivacea_ and _Ophiocantha bidentata_. [28] PAGE 31. _Brisinga elegans._