Sexual Organs

(also reproductive organs), the organs of sexual reproduction.

In animals. Sexual organs in animals include sexual glands, or gonads (testes and ovaries), gonaducts (seminal ducts and oviducts), accessory organs, and copulatory organs. Sexual cells, or gametes (ova in the ovaries and spermatozoa in the testes), form and mature in the gonads. Gametes are eliminated from the organism through oviducts, seminal ducts, and in some animals, through excretory ducts.

In most flatworms, the vitellaria, or modified ovaries, form yolk cells that are incapable of developing but are rich in nutrients for the embryo. Accessory organs include various glands; in the male they secrete a substance that acts as a physiological medium for spermatozoa and intensifies their activity. In the female they secrete substances that form the vitelline membrane. Other accessory organs are the spermatheca, which is a storage reservoir for spermatozoa, and seminal bursae. Female viviparous animals have all the sexual organs necessary for producing live young.

Most animal species are dioecious, although some species are hermaphroditic. Sponges and some turbellarians do not have gonads; their sexual cells are scattered in the parenchyma and elminated through the mouth opening or a rupture in the body wall. Coelenterates have small gonads in specific areas of the body. Most flatworms have a hermaphroditic reproductive system, with various accessory organs. Lateral saclike gonads with short and simple ducts are recurrent along the body of nemer-tines. Roundworms are dioecious and have a comparatively simple reproductive system. The gonads are on the walls of the coelom in annelids, mollusks, echiuroids, sipunculids, bryo-zoans, and brachiopods. Here, the gonaducts are called coe-lomoducts and are in the form of canals that are not connected to the gonads. Eggs and sperm first enter the coelom and then pass into the infundibulum of the coelomoduct, which opens to the outside. Arthropods have gonads and gonaducts that are products of the coelom walls and coelomoducts, respectively. In echinoderms, the gonads are usually arranged in five-ray symmetry. Only sea cucumbers have unpaired gonads. In acraniates, such as lancelets, the gonads consist of small paired ductless sacs adjacent to the walls of the atrium. Sperm is released into the atrium through a short duct that develops at sexual maturity, and eggs are released through a fissure in the wall of the ovaries and atrium.

In vertebrates, sexual organs are connected to excretory organs and form a single urogenital system. Cyclostomes have only one ductless gonad, which was originally paired. Eggs and sperm are eliminated from the body cavity through genital pores on the animal’s body. The gonads of gnathostomes are in the body cavity and are usually paired. Eggs and sperm are generally eliminated from the body through excretory canals that open into the cloaca. In neopterygians, the cavity containing the ovaries and testes usually extends into a short canal that opens to the outside.

Some selachians, chimaeroids, and teleosts have a copulatory organ. In amphibians, the ovaries and testes are suspended on mesenteries from the dorsal wall of the abdominal cavity. Mature eggs are eliminated from the body cavity through long winding oviducts that open into the cloaca. Sperm is eliminated through seminal ducts that also open into the cloaca. The caecil-ians have special copulatory organs.

In reptiles, paired gonads open by way of gonaducts into the cloaca. The seminal vesicle forms the terminus of the seminal duct. In some lizards and snakes, the ovaries and oviducts are asymmetrical. Birds have only a left ovary and left oviduct because of the large size of their eggs. Copulatory organs are developed in all reptiles, except tuataras, and in some birds, including the Ratitae, Tinamiformes, Anseriformes and some Galliformes.

In mammals, the ovaries are in the posterior section of the abdominal cavity. In monotremes, paired ovaries and oviducts open into the urogenital sinus. The posterior part of each oviduct widens into the uterus. In marsupials, each oviduct is divided into three sections: the anterior section, or fallopian tube; the middle section, or uterus; and the posterior section, or vagina. The vagina opens into the urogenital sinus. In eutherians, the posterior ends of the oviducts fuse to form an unpaired vagina that opens into the urogenital sinus. In higher eutherians, the urogenital sinus is considerably reduced and is called the vestibule of the vagina. The uterine sections of the oviducts take many forms; they are completely separated in elephants and many rodents (duplex uterus); they are partly fused together in their posterior sections in some rodents, predators, and swine (uterus bilocularis); they are almost completely fused together in some predators, ungulates, insectivores, and whales (uterus bi-cornis); and they are completely fused together in bats, monkeys, and humans (uterus simplex).

The testes remain in the posterior section of the abdominal cavity throughout the lifespan of monotremes, some edentates, insectivores, whales, elephants, and sirenians. In marsupials and most placentals, the testes drop from the body cavity into the scrotum, which is situated in front of the penis in marsupials and behind the penis in placentals.

In man.MALE SEXUAL ORGANS. The male internal sexual organs include the testes and its appendages, deferent ducts, seminal vesicles, the prostate gland, and bulbourethral glands (glands found in the bulbous part of the urethra). The external sexual organs are the scrotum and the penis. The epididymis lies posterior to its testis and stores spermatozoa; its canal turns into a deferent duct 45–50 cm in length. As part of the spermatic cord, this duct proceeds from the testis, through the inguinal canal, and into the abdominal cavity. It then descends into the pelvis minor and joins the efferent duct of the seminal vesicles behind the urinary bladder to form the ejaculatory duct, which opens into the urethra. The seminal vesicles are behind and below the urinary bladder and secrete a viscous albuminous fluid that dilutes semen and facilitates the movement of spermatozoa. The paired bulbourethral glands are each the size of a pea and are at the root of the penis. During ejaculations, they secrete a substance into the urethra. The penis is the organ of urinary excretion and ejaculation of semen. It has a head, or glans, a body, and a root. The root attaches the penis to the pubic bone. The external orifice of the urethra is on the glans. The penis consists of spongioid corpora cavernosa (one lower and two upper). The urethra passes through the lower one. The corpora cavernosa are covered with tunica albugínea and a thin skin. Near the glans, the skin forms a fold called the prepuce.

FEMALE SEXUAL ORGANS. The female internal sexual organs are found in the cavity of the pelvis minor. They include the ovaries, oviducts (uterine tubes), uterus, and vagina. The external sexual organs are the vestibule of the vagina, the labia majora pudendi, the labia minora pudendi, and the clitoris. The labia majora are two folds of skin that circumscribe the rima pudendi and extend from the pubic tubercle to the perineum. The labia minora are between the labia majora. The vestibule of the vagina is between the labia minora. The external orifice of the urethra and the vaginal entry behind it open into the anterior portion of the vestibule. The clitoris consists of two corpora cavernosa. It has a glans in the anterior portion of the vestibule, two crura attached to the pubic bone, and a body. The glans of the clitoris and the labia minora are rich in nerve endings. The paired vestibular glands are the size of a pea and are located at the base of the labia minora. Their secretion moistens the walls of the vestibule.

Among the developmental anomalies of the sexual organs are cryptorchidism, hypospadias, epispadias, duplex uterus, uterus bicornis, and vagina septa.

In plants. Sexual organs are unicellular in most lower plants. In higher plants, they are multicellular and have a wall of external sterile cells. The sexual organs of most isogamous plants do not differ from other cells, although they are larger and multicellular in some plant forms, for example, in some brown algae. In oogamous plants, the antheridium is the male sexual organ. The female sexual organ is the oogonium in most algae and some fungi, the carpogonium in red algae, and the archegonium in bryophytes, ferns, and gymnosperms. Spermatozoids, or non-flagellated spermatia, form in the antheridia. (The latter form in such plants as red algae.) Egg cells form in the female sexual organs.

Oogonia are unicellular except in charophytes. The bottle-shaped bicellular carpogonium consists of a venter, which contains the egg cell, and a long narrow trichogyne, which traps the spermatia. The venter of the archegonium contains the egg and ventral canaliculated cells, while canaliculated neck cells are in the narrow neck. In most ascomycetes, the multinucleate protoplast of the antheridium overflows through the trichogyne, or upper part of the archicarp, and into the ascogonium, or basal part of the archicarp, which also contains a multinuclear protoplast. The sexual process is called gametangiogamy. In zygomycetes, two gametangia fuse that have protoplasts that are not differentiated into gametes. Some unicellular algae, such as Chlamydomonas, are themselves apparently converted into gametangia and form gametes. In algae that reproduce by conjugation, the protoplasts of two cells fuse and there are no special sexual organs. The Spirogyra is an alga that reproduces by conjugation. There are also no special sexual organs in basidiomycetes, in which the cells of the primary mycelia fuse.

In the course of evolution, seed plants lost their male sexual organs. Gymnosperms, such as Gnetum and Welwitschia, and angiosperms lost both their female and male sexual organs. In these angiosperms and gymnosperms, spermatozoids or non-flagellated sperm form in male gametophytes or pollen grains, and egg cells form in female gametophytes. The female gametophytes are embryo sacs in angiosperms and primary endosperm cells in gymnosperms.