diversity of animal

  The diverse appearance ofanimals is mostly superficial; the bewildering variety of known forms, sometruly bizarre, can be assorted among a mere half-dozen basic body plans. Theseplans are established during the embryonic stages of development and limit thesize and complexity of the animals. Symmetry, number and relative developmentof tissue layers, presence and nature of body cavities, and several aspects ofearly development define these fundamental modes of organization.

social levels of organization

  Large size is oftencompetitively advantageous but unobtainable by many animals because ofconstraints of basic body plan. Intrinsically small animals sometimes becomelarge in the same way that protozoans evolved into metazoans: they multiply thenumber of individuals by asexual reproduction (thus maintaining the samegenotype) and remain attached, with the option that individuals can be modifiedduring their development for a specialized function. This type of asexualsociality forms the colonoids of sponges, coelenterates, bryozoans,hemichordates, and tunicate chordates, all of which were primitivelysmall, sessile filter feeders. Staying together after asexual budding of newindividuals gave a competitive edge to monopolizing available space. With slight modifications sothat all individuals in the colony could share equally in the gains, theselarger entities had the energy reserves necessary to outcompete smaller organisms for space.This type of sociality has evolved in ways that complicate the definition ofindividuality. For instance, Portuguese men-of-war and their kin (somehydrozoan coelenterates) look and act like single individuals, yet theircomponents develop as genetically identical units, each homologous to a whole jellyfish or polyp. It is a questionwhether such an animal should be considered one individual or many.

  A different typeof sociality emerged among mobile complex animals that can individually attainlarge size. In fact, the largest known living animals, the whales andelephants, comprise two of a very fewmammalian orders that contain only social species. The pattern of evolution on Earth has favouredsociality in the smallest and the largest (mostly vertebrates) of animals, albeit for differentreasons. The smallest seek the advantages of being large, as protozoans did toform the first animals. The large animals can communicate; they spread out tofind food, which all can share, and they protect one another. Among the socialgroups of large animals, only humans have differentiated their functionsto such an extent that their societies begin to behave as individuals.

  Insect societies show behaviours halfway between societies based on genetically identicalmembers and those created by genetically different individuals; such propertieslargely reflect their intermediate degree of genetic relatedness. Insects aremore cooperative and show a greater degree of altruism than is true of vertebrate societies.

form and function

  To stay alive,grow, and reproduce, an animal must find food, water, and oxygen, and it must eliminate the waste products ofmetabolism. The organ systems typical of all butthe simplest of animals range from those highly specialized for one function tothose participating in many. The more basic functional systems are treatedbelow from a broadly comparative basis.