Sexual maturation

After the embryo or larva is transformed into a juvenile form, all that remains for the individual is maturation of its sexual reproductive systems, accompanied or followed by mei-otic maturation of the gametes to form fully functional and fertilizable sperm and oocytes. The mechanisms for maturation vary widely among animal phyla, but are especially diverse among the lower metazoans. Such phyla as placozoans and sponges have no discernible gonads; the gametes simply form out of previously undifferentiated somatic cells. In such others as the diploblastic cnidarians, the gametes develop from specialized cells within one of the germ layers, but no other gonadal tissues are present. At the other end of the scale of complexity, such animals as roundworms and flatworms develop elaborate gonads and ducts; these structures include discrete testes with sperm ducts, discrete ovaries with oviducts, and even numerous types of specialized reproductive glands. In some respects, the parasitic flatworms have the most structurally sophisticated reproductive systems in the animal king dom, even though as a group they are considered primitive organisms.

Another variation that occurs among the lower metazoa relates to reproductive seasonality. While some groups, such as tapeworms and flukes, reproduce continually and thus retain all of their sex organs, such others as hydroids and turbel-larian flatworms reproduce only temporarily, but repeatedly. These organisms often lose their reproductive organs during their nonreproductive periods and redevelop them during the next mating season.

Structural maturation of the sex organs and gametes is often accompanied by certain changes in behavior as well as the development of special structures that are not gonadal but are nonetheless related to reproduction. The most obvious behavioral changes involve mate-seeking and copulatory behaviors. Structural changes include the development of special genitalia for coupling. The latter includes the copulatory cirrus (flexible penis) of parasitic flatworms and the copulatory spicules of many roundworms. Other related behavior may include various forms of brooding or other parental care strategies. While parental care is not generally as common or as well developed among lower metazoans as in higher animals, some examples do exist. Among the lower metazoans, most of these are more structural than behavioral. Examples include the maintenance of amphiblastula larvae within specially adapted radial canals of calcareous sponges; the development of special egg-enclosure organs in several tapeworms; the retention of fully-formed juveniles within the uterus of some roundworms; and the retention of successive generations as colonies in many hydroids and other cnidarians.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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