In addition to _Gasterosteus aculeatus_, whom we have above alluded to, there is another species, _G. spinachia_, or Fifteen-Spined Stickleback, which is also an inhabitant of rock-pools by the sea-shore, but unlike the first-mentioned, is never found in fresh water. Both species possess one peculiarity in common, a description of which will form an appropriate conclusion to this chapter. I allude to their nest-building habits, which has only of late years been proved to exist, although Aristotle has recorded the same fact regarding a fish (_Phycis_) in the Mediterranean Sea, which was known to make a nest and deposit its spawn therein. The duties of mason and architect are invariably undertaken by the male Stickleback. His materials are of course very limited, still his labours are skilfully and even artistically performed. Having chosen a suitable spot as a foundation for his house, he collects some delicate sea-weeds, gravel and sand, and with these materials, aided by a glutinous fluid which is given off from his body, the house is built. When completed, and not before, he seeks out his mate, and invites her to take possession of her newly formed home. If she shows any affectation or coquetishness, he does not hesitate to nip hold of her tail, and urge her forward by equally expressive signs. Soon, like a dutiful little pet, she enters, and having deposited spawn, retires again, leaving her lord and master to guard the casket and its living treasure. This task, though extremely arduous, he adopts with pride and gratification. How so small a creature can bear up so long under such a state of apparent excitement appears marvellous. His assiduity is most extraordinary. By night he rests beside the nest, and by day, if he can possibly hinder it, he allows nothing to approach. When there are other members of the Stickleback family in the aquarium, numerous combats are sure to ensue, for as the young and transparent offspring of one fish are deemed a great dainty by the non-parental body, the latter invariably endeavour to satisfy their cannibal propensities at the harrowing expense of their neighbours. When the spawn are hatched, fresh care devolves upon the parent, in order to keep them within the nursery, and protect them from the greedy mouths of the larger fish, always on the look-out for tit-bits. Should one of the little fishlings stray beyond the prescribed bounds, the watchful parent darts after it, and in an instant his jaws close over the wanderer apparently for ever, but in fact only for a time, for swimming quickly back the old fish puffs out the straggler into its nest lively and uninjured. CHAPTER XXV. On the Formation of an Aquarium, &c.