In many diverse species there is clear evidence of some form of parental solicitude. The spotted goby, or pole-wing, for instance, a fish which is found in the Thames, is a nest-builder. An old cockle-shell is skilfully utilised to form the nursery. The shell is placed with its cavity turned downwards, beneath it the soil is removed and then the earth-walls are cemented together with a secretion from the skin of the parents. Access to the nest is gained by a cylindrical tunnel, and the whole nursery is covered and concealed by loose sand. Again it is the father who mounts guard over the eggs; his vigil lasts for about nine days.[24] There are many instances of nursery building undertaken by fish parents. Agassiz[25] records a case in which an elaborate nest formed of knotted weeds is made by a certain fish, known as _Chironectes_. This rocking fish-cradle is carried by both parents and is a kind of arbour, affording protection and afterwards food for its living freight.[26] A remarkable nest is built by the American bow-fin (_Amia calva_), found in the eastern states of North America. Both the mother and the father work together to construct the nursery, which is formed by a large circular area cleared among the weedy shallows; these intelligent parents actually bite through the stems of all the plants that they cannot break or push aside. In the pool that is thus made the eggs are placed by the mother and fertilised by the father; the young develop with remarkable rapidity and hatch out in about eight days from time of laying. The family is then jealously guarded by the father, who herds the children—often numbering as many as a thousand individuals[27]—by circling around and above them in untiring watchfulness. Another remarkable nest is that of the eel-like _gymnarchus_ of the Nile; a huge floating nursery is made of grasses, measuring some two feet long and a foot broad. Within this nest some thousand eggs are laid, and as soon as they are deposited by the mother, the father mounts guard, defending them, and afterwards the young, with great ferocity. Some fishes’ nests, like those made by the frogs, are constructed of foam. M. Carbonnier gives the case of a Chinese butterfly fish in his private aquarium in Paris. The male fish constructed a large nest of froth, fifteen to eighteen centimetres horizontal diameter and ten to twelve centimetres high: this he did by a curious sucking and expelling air which formed the mucus in his mouth into a white foam. When the nest was thus prepared the female was induced to enter. I do not know whether the father’s duty was continued after this point.[28]