Most animal embryos look rather similar to one another up through the stage of gastrulation. It is during the important stages of postembryonic development, however, that the characteristic features of specific phyla, classes, and orders finally emerge. For this reason, much of postembryonic development is said to involve morphogenesis, or the establishment of the animal's definitive body form. Along with the completion of form comes the establishment of function, so that the end result is a fully functional animal. For some species, this fully functional individual will be a juvenile, which resembles an adult in form but lacks a mature reproductive system. Development that proceeds from embryo to juvenile with no intervening stage is known as direct development. Direct development occurs in some lower metazoans, including the nematodes, gnathostomulids, rotifers, and gastrotrichs. In contrast, most lower metazoans undergo indirect development, in which a larval stage is inserted between the embryo and the juvenile or adult.
A larva is a fully functional animal, generally feeding and moving about independently. The larval form of a given species is generally as characteristic of the species as the adult, and may complete some critical parts of the life history strategy for its species. The most common task of larvae is longdistance migration in order to colonize new environments for the species. This phenomenon, known as planktonic dispersal, is especially critical in marine species whose adult forms have limited or no mobility, such as corals, sponges, ribbon-worms, and polyclad flatworms. Larvae may also make use of food resources that differ from those needed by the parent, thus avoiding competition within the species. Because of the critical and distinctive attributes of larvae, their formation is frequently referred to as larvigenesis, and represents a discrete (separate) stage of postembryonic morphogenesis.
By definition, larvae and adults are dissimilar in structure and function. For this reason, the transition from larva to adult requires radical changes in the morphological, behavioral, and physiological characteristics of the animal. This transformation between successive postembryonic forms is known as metamorphosis. As with cleavage and gastrulation, the exact mechanism of metamorphosis varies widely among different lower metazoan phyla. In all, it involves the loss of some specifically larval structures and the development of new adult structures from groups of undifferentiated cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.