In the case of the hermit-crab and the sea-anemone there is no doubt of the mutual advantage derived from their communal life. But this mutual advantage is not so obvious in some cases of commensalism, where indeed most or all of the advantage often seems to lie with one of the animals, while the other derives little or none, but on the other hand suffers no injury. For example, "small fish of the genus _Nomeus_ may often be found accompanying the beautiful Portuguese man-of-war (_Physalia_) as it sails slowly about on the ocean's surface. These little fish lurk underneath the float among the various hanging thread-like parts of the man-of-war which are provided with stinging cells. They are protected from their enemies by their proximity to these stinging threads, but of what advantage to the man-of-war their presence is is not understood." Similarly in the nests of the various species of ants and termites many different kinds of other insects have been found. "Some of these are harmful to their hosts, in that they feed on the food-stores gathered by the industrious and provident ant, but others appear to feed only on refuse or useless substances in the nest. Some may be of help to their hosts by acting as scavengers. Over one thousand species of these myrmecophilous (ant-loving) and termitophilous (termite-loving) insects have been recorded by collectors as living habitually in the nests of ants and termites." =Parasitism.=--TECHNICAL NOTE.--As examples of temporary external parasites find and examine fleas and ticks on dogs and cats, red mites on house-flies and grasshoppers (at the bases of the wings), etc. As examples of permanent external parasites find bird-lice on pigeons or domestic fowls or on other birds. Note the absence of wings and the peculiarly modified body shape of these parasites. Examine a bird-louse under the microscope; note the absence of compound eyes (it has simple eyes) and absence of wings; note bits of feathers, its food, in stomach, showing through the body. Find, as examples of internal parasites, intestinal worms or flukes. Examine trichinized pork to see _Trichinæ_ in muscles. Examine preserved specimens of tapeworms. Collect pupæ of some common butterfly or moth and keep them in the schoolroom until either the butterflies or ichneumon flies issue. Some will surely be parasitized, and yield ichneumon flies (parasites) instead of a butterfly. As examples of degeneration by quiescence examine barnacles (found on outer rocks of seashore at low tide; easily obtained as preserved specimens by inland schools) and the females of scale-insects. These insects may be found on oleanders (the black scale, _Lecanium oleæ_) or fruit-trees (the San Jose scale, _Aspidiotus perniciosus_). Note the great degeneration of the adult female of the San Jose scale; it has no eyes, antennæ, wings, or legs.