The common garden slugs, the cousins of the snails, are well-known forms. They generally remain concealed during the day, coming out at night, and often doing much damage to vegetation which is largely laid to birds. I have kept many of them, and they offered an extremely interesting study. They secrete a remarkable amount of mucus, which they use in descending from a tree, just as a spider does its silk thread. The mucus exudes from the foot, passes along to the tail, when it is attached to the twig. This accomplished, the slug boldly launches itself into space, the thread becoming more and more attenuated, until finally, when the slug is near the ground, it is exceedingly fine. Nearly all our common slugs descend from trees in this manner,--quite a contrast to the slow, tedious ascent. The amount of mucus that can be taken from them is remarkable; and that it is also protective will be evident to any one who may experiment with them. One genus, _Phosphorax_, found at Cape Verde, and, according to Duncan, at Teneriffe, has a luminous pore on the posterior border of the mantle. One species only is known, _P. noctilucus_; and its light has not, that I am aware, been made the object of any extended investigation. The highest forms of the _Mollusca_, the Cephalopods, cuttle-fishes, are probably at times luminous. I have noticed what I presumed was a delicate, sensitive, luminous glow about an _Octopus_ in a semi-darkened tank, but I am not satisfied to make the statement as fact. These forms are so remarkable for the waves of color that pass over them, and which seem to make them transparent, that one could readily be deceived. The little _Cranchia_ (Plate IV., Fig. 2) is a light-giver, its phosphorescence having been distinctly observed. It is an ally of the giant squids, which have been found fifty-five feet in length, and which, if luminous like their pygmy relative, would present a marvellous spectacle, darting veritable living arrows through the depths of the sea. Giglioli refers to the phosphorescence of _Loligo saggitatus_, and to that of several small Octopods observed by him at Callao and Valparaiso. Their bodies gave out a pale whitish light, uniformly distributed. [Illustration: PLATE IX. _c_ _b_ _a_ _f_ _e_ _d_ LUMINOUS BEETLES, ETC. _a._--_Lampyris splendidula_--male. _b._-- „ „ --female. _c._-- „ „ --larva. _d._--_Lampyris noctiluca_ --male. _e._-- „ „ --female. _f._-- „ „ --larva.] CHAPTER VII. LIGHTNING-BUGS.