The bullheads or miller's thumbs, those quaint big-headed beasts which divide with the sticklebacks the polite attentions of ingenious British youth, are also nest-builders, and the male fish are said to anxiously watch and protect their offspring during their undisciplined nonage. Equally domestic are the habits of those queer shapeless creatures, the marine lump-suckers, which fasten themselves on to rocks, like limpets, by their strange sucking disks, and defy all the efforts of enemy or fishermen to dislodge them by main force from their well-chosen position. The pretty little tropical walking-fish of the filuroid tribe--those fish out of water--carry the nest-making instinct a point further, for they go ashore boldly at the beginning of the rainy season in their native woods, and scoop out a hole in the beach as a place of safety, in which they make regular nests of leaves and other terrestrial materials to hold their eggs. Then father and mother take turns-about at looking after the hatching, and defend the spawn with great zeal and courage against all intruders. I regret to say, however, there are other unprincipled fish which display their affection and care for their young in far more questionable and unpleasant manners. For instance, there is that uncanny creature that inserts its parasitic fry as a tiny egg inside the unsuspecting shells of mussels and cockles. Our fishermen are only too well acquainted, again, with one unpleasant marine lamprey, the hag or borer, so called because it lives parasitically upon other fishes, whose bodies it enters, and then slowly eats them up from within outward, till nothing at all is left of them but skin, scales, and skeleton. They are repulsive eel-shaped creatures, blind, soft, and slimy; their mouth consists of a hideous rasping sucker; and they pour out from the glands on their sides a copious mucus, which makes them as disagreeable to handle as they are unsightly to look at. Mackerel and cod are the hag's principal victims; but often the fisherman draws up a hag-eaten haddock on the end of his line, of which not a wrack remains but the hollow shell or bare outer simulacrum. As many as twenty of these disgusting parasites have sometimes been found within the body of a single cod-fish.