fl. are transformed into fishing-lines that catch food, and at the same time are so well provided with stinging-cells as to effectively keep off enemies. There are also digestive individuals, which devour and digest the animals caught by the fishing-lines. Some members are reduced to tentacles, ministering to the sense of touch (and possibly smell); others again are in the form of protective plates, covering and sheltering adjacent individuals. And there are also egg-producing members of the colony which may be liberated as little medusæ, thus promoting dispersal. In different species there are considerable variations in detail, and the actual arrangements in one case (Stephalia corona) are shown in fig. 1093. pr f.l. d p. pr. COLONIAL Moss POLYPES (POLYZOA). With a single exception the mem bers of this group budding, after the Fig. 1092.-Diagram of various in Zoophytes, dividuals in a colony of Compound Jelly-Fish (Siphonophora), central cavity of colony indicated in black. A, Float; s, swimming-bell; d, d, digestive individuals; f., branched fishing-line: pr, protective individual: P, P. P. various forms of pro pagative individual; t, tentacle. for fl. Fig. 1093.-A Compound Jelly - Fish (Stephalia corona), reduced. A, Float; mis-s, swimming-bell; g, gas-conducting individual; d, small digestive individual; m.d., which they are large central digestive individual; ¿, ten tacle;, group of small propagative in dividuals. of the members of such a colony may be greatly specialized for various purposes (fig. 1094), e.g. they may be modified into rounded receptacles (ovicells) in which the eggs develop till the time of hatching. In certain species there are bird's-head individuals, which execute vigorous snapping movements, the object of which is extremely doubtful. Cleanliness is possibly promoted, or perhaps the attacks of small parasitic forms may thus be warded off. It has also been noticed that the powerful jaws often succeed in catching little worms, crustaceans, &c., apparently holding them tenaciously till they die and decompose. The suggestion has been made that the decayed fragments of these victims are carried by A ciliary action into the mouths of the unmodified members of the colony, thus serving as food. But there is no definite proof that such is the case. Individual Moss-Polypes may also undergo still greater modification into whip-like threads that actively lash about in all directions. Cleanliness and defence have here again been suggested as the ends to be served, and cases have been observed where the action is so vigorous as to move the entire colony about. That the surrounding water should be thoroughly stirred up is probably advantageous with reference both to feeding and breathing. The only thing, however, that we definitely know about these curious structures is, that they have been evolved from bird's-head individuals by suppression of the "head", and prolongation of the "lower jaw" into a slender filament. B Gut COLONIAL TUNICATES (UROCHORDA). — The formation of colonies is clearly related to powers of increase by means of budding or fission, and consequently all the members of certain animal groups devoid of such powers, e.g. Arthropods and Molluscs, are non-colonial. This is also true for the vast majority of Backboned Animals, the most notable exception being afforded by many species of the lowly and degenerate forms known as Sea-Squirts, Tunicates, or Ascidians. Most of these are fixed to some firm object when adult, and their sedentary life has no doubt had much to do with the degeneration they have undergone (see vol. iii, p. 421). A good many Tunicates are non-colonial or "solitary", but others bud to produce colonies of various shape. In such species the individual members may be borne on a creeping stem and clearly marked off from one another, much as in a hydroid zoophyte, or the association may be much more intimate. In the latter case the individuals are sunk within a sort of common body (like the cœnosarc of colonial corals), and there is a continuous protective investment or common test. A good instance is afforded by Botryllus (fig. 1095), to be found at low tide on our coasts as a sort of bluish encrustation on sea-weeds and stones. It Fig. 1094.-Parts of Colonies of Moss - Polypes, enlarged. A, Bugula; B, B, B, bird's-head individuals (one holding a small worm); E, ovicell; T, tentacles of an ordinary individual. B, Scru 'solitary ", but pocellaria, showing three whip-shaped individuals. presents at regular intervals a number of flower-like markings, each of which is made up of a circlet of individuals, with their mouths near the tips of the "petals". A small hole in the centre of the flower leads out of a cavity ("common cloaca ") into which are discharged the waste products of the members of the group. The surface population of the sea is also partly made up of colonial Tunicates. The Salps, for instance, present two stages in their life-history, one of which propagates by budding, the other by eggs (see vol. iii, p. 422). A large number of the latter stage Α B Fig. 1095.-A, Fire-Cylinder (Pyrosoma) in side view, the small rounded areas are the mouths of members of the colony. B, Open end of same. c, Small colony of Botryllus, showing circlets of individuals. D, Two circlets, enlarged. are connected together when young into "chains", which may be regarded as temporary colonies. These ultimately break up into their constituent members. A notable example of a permanent freeswimming Tunicate colony is afforded by the Fire - Cylinder (Pyrosoma, fig. 1095), abundant in the Mediterranean and elsewhere, and giving off a bright phosphorescent light, possibly as a means of protection. The colony is shaped like a hollow cylinder which, in one large species (Pyrosoma gigantea), may be as much as 5 feet long, and possesses a contracted aperture at one end, the other being closed. The external surface is covered with pointed projections of the firm test. The small but very numerous individuals are imbedded transversely in the wall of the cylinder, their mouths being external. The large central cavity receives all the products of waste, and is comparable to the common cloaca of a Botryllus circlet. The size of the colony is augmented by budding, and eggs are also produced, which develop into minute colonies that are liberated into the surrounding water, there to grow to their full size. CHAPTER LXII ASSOCIATION OF ORGANISMS-SOCIAL BACKBONELESS ANIMALS (INVERTEBRATA). Many animals are social or gregarious, and in such cases division of labour between the members of a community is more or less perfectly exemplified. The mere fact that many individuals of the same species live together in the same place does not entitle them to be termed social, in the sense here intended, unless there is some sort of co-operation which benefits the animals living together. It would hardly be justifiable, for example, to describe oysters, cockles, or star-fishes, as colonial animals. But even here the species may be benefited, e.g. weakly individuals are weeded out in the keen struggle for existence, so that the stock becomes increasingly healthy and strong. And from such casual kinds of association communities of very complex kind have gradually been evolved, the benefits to be derived from division of labour between individuals giving various species a greatly improved chance of survival in the competition with other species. But here a qualification must be made. For in the world of organisms, by the irony of fate, an extreme penalty attaches to elaborate specialization resulting from adjustment to the exigencies of a certain set of conditions. The surroundings of animals (and plants) are constantly changing, and if these alter suddenly, as they are liable to do, a well adapted species may be unable to adjust itself with sufficient rapidity to the new order of things, and hence be doomed to extinction, while simpler but more plastic forms may survive. Many lowly organisms have endured through countless ages, while others of more complex kind have quickly succumbed to rapid alterations of their environment at a time when their continued dominance seemed most certain. Innumerable instances of this far-reaching principle are to be drawn from the geological record, which preserves for us the past history of the globe. The most interesting cases of the social habit are to be found among Insects and Backboned Animals, which it will be well to consider separately. SOCIAL INSECTS (INSECTA) Some of the most remarkable facts in natural history have been made known by those who have studied the complex communities existing among various species of Membrane-Winged Insects (Hymenoptera) and Net-winged Insects (Neuroptera). The extent to which division of labour is carried varies greatly in different cases, so that we are able to get some notion of the successive stages which have marked the evolution of the social habit. SOCIAL MEMBRANE-WINGED INSECTS (HYMENOPTERA). — The most salient point distinguishing highly organized communities of Bees, Wasps, and Ants is the presence of a varying number of "castes" or kinds of individual. In the Honey-Bee (Apis mellifica), for instance, a hive contains not only males (drones) and an egg-producing female (queen), but also numerous "workers", which are a second kind of female, having nothing to do with the production of eggs, but, as their name indicates, labouring for the benefit of the republic. And in other cases there may be more than three castes, as we shall see in the sequel. Worker honeybees differ markedly in size and structure from the queen, as the result of a long course of evolution, and it will be desirable to begin with simpler communities, where such sharp distinctions do not exist. BEES. Some account has already been given of CarpenterBees, Mason-Bees, and Leaf-cutting Bees, solitary forms in which the female not only lays eggs but also makes and stores a nest (see vol. iii, p. 390). These and many other non-colonial insects exhibit very elaborate adaptations to their surroundings, and it would be a mistake to consider them as necessarily lower in the scale than colonial forms, which have evolved on entirely different lines. Here, as in all other cases, success in the struggle for existence may be attained in widely different ways. From the purely solitary life led by the bees above-mentioned, we pass to a curious case described by Fabre (from his observations in South France), where a certain amount of co-operation is |