Certain tubicolar annelids, whose skin furnishes abundant mucus which does not become sufficiently hard to form an efficacious protection, utilize it to weld together and unite around them neighboring substances, grains of sand, fragments of shell, etc. They thus construct a case which both resembles formations by special organs and manufacture by the aid of foreign materials. The larvæ of Phryganea, who lead an aquatic life, use this method to separate themselves from the world and prepare tubes in which to dwell. All the fragments carried down by the stream are good for their labors on condition only that they are denser than the water. They take possession of fragments of aquatic leaves, and little fragments of wood which have been sufficiently long in the water to have thoroughly imbibed it, and so become heavy enough to keep themselves at the bottom, or at least to prevent them from floating to the surface. It is the larva of Phryganea striata which has been best studied; those of neighboring species evidently act much in the same way, with differences only in detail. The little carpenter stops a fragment rather longer than his own body, lies on it and brings it in contact with other pieces along his own sides. He thus obtains the skeleton of a cylinder. The largest holes are filled up with detritus of all kinds. Then these materials are agglutinated by a special secretion. The larva overlays the interior of its tube with a covering of soft silk which renders the cylinder water-tight and consolidates the earlier labors. The insect is thus in possession of a safe retreat. Resembling some piece of rubbish, it completes its metamorphosis in peace, undisturbed by the carnivora of the stream. Between the beings whom nature has endowed with a shelter and those who construct it by their own industry, we may intercept those who, deprived of a natural asylum and not having the inclination or the power to make one, utilize the dwellings of others, either when the latter still inhabit them, or when they are empty on account of the death or departure of the owner. In the interior of the branchial chamber of many bivalvular mollusca, and especially the mussel, there lives a little crustaceous commensal called the pea-crab (Pinnoteres pisum). He goes, comes, hunts, and retires at the least alarm within his host’s shell. The mussel, as the price of its hospitality, no doubt profits by the prizes which fall to the little crab’s claws. It is even said that the crab in recognition of the benefits bestowed by his indolent friend keeps him acquainted with what is passing on around, and as he is much more active and alert than his companion he sees danger much further away, and gives notice of it, asking for the door to be shut by lightly pinching the mussel’s gill.