Penis

A penis (: penises or penes) is a male sex organ that is used to inseminate female or hermaphrodite animals during copulation. Such organs occur in both vertebrates and invertebrates, but not in all male animals.

The term penis applies to many intromittent organs, but not to all. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. Even within the Vertebrata, there are morphological variants with specific terminology, such as hemipenes.

Etymology
The word "penis" is taken from the Latin word for "tail". Some derive that from Indo-European *pesnis, and the Greek word πέος = "penis" from Indo-European *pesos. Prior to the adoption of the Latin word in English, the penis was referred to as a "yard". The Oxford English Dictionary cites an example of the word yard used in this sense from 1379, and notes that in his Physical Dictionary of 1684, Steven Blankaart defined the word penis as "the Yard, made up of two nervous Bodies, the Channel, Nut, Skin, and Fore-skin, etc." According to Wiktionary, this term meant (among other senses) "rod" or "bar".

As with nearly any aspect of the body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock". See WikiSaurus:penis for a list of alternative words for penis.

The Latin word "phallus" (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe representations, pictorial or carved, of the penis.

Evolution and function
The external genital organs appeared in the Devonian, about 410 million years ago, when tetrapods began to abandon the aquatic environment. In fact, the necessity to overcome the absence of a liquid phase in which to release the gametes was achieved through the transition to internal fertilization.

Among amniotes, the development of an erectile penis occurred independently for mammals, squamates (lizards and snakes), testudines (turtles), and archosaurs (crocodiles and birds).

Over time, birds have lost this organ, with the exception of Paleognathae and Anseriformes.

The penis is an intromittent organ used to transfer sperm into the female genital tract for potential fertilization and, in the case of placentals, also for the excretion of urine. The penises of different animal groups are not homologous with each other, but were created several times independently of each other in the course of evolution.

An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. During ejaculation, a series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis. Ejaculation is usually accompanied by orgasm.

The last common ancestor of all living amniotes (mammals, birds and reptiles) likely possessed a penis.

Birds
Most male birds (e.g., roosters and turkeys) have a cloaca (also present on the female), but not a penis. Among bird species with a penis are paleognaths (tinamous and ratites) and Anatidae (ducks, geese and swans). A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall (in ducks) and being erected by lymph, not blood. It is usually partially feathered and in some species features spines and brush-like filaments, and in a flaccid state, curls up inside the cloaca.

Mammals
As with any other bodily attribute, the length and girth of the penis can be highly variable between mammals of different species. In many mammals, the penis is retracted into a prepuce when not erect. Mammals have either musculocavernous penises, which expand while erect, or fibroelastic penises, which become erect by straightening without expanding. Preputial glands are present in some prepuces. The penis bears distal part of the urethra in placental mammals. The perineum of testicond mammals separates the anus and the penis.

A bone called the baculum is present in most placentals but absent in humans, cattle and horses.

In mammals, the penis is divided into three parts:


 * Roots (crura): these begin at the caudal border of the pelvic ischial arch.
 * Body: the part of the penis extending from the roots.
 * Glans: the free end of the penis.

The internal structures of the penis consist mainly of cavernous, erectile tissue, which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae).

Canine penises have a structure at the base called the bulbus glandis. During copulation, the spotted hyena inserts his penis through the female's pseudo-penis instead of directly through the vagina, which is blocked by the false scrotum. The pseudo-penis closely resembles the male hyena's penis, but can be distinguished from the male's genitalia by its greater thickness and more rounded glans. Domestic cats have barbed penises, with about 120–150 one millimetre long backwards-pointing spines.

Marsupials usually have bifurcated penises that are retracted into a preputial sheath in the male's urogenital sinus when not erect. Monotremes and marsupial moles are the only mammals in which the penis is located inside the cloaca.

Reptiles
Male turtles and crocodiles have a penis, while male specimens of the reptile order Squamata, which are snakes and lizards, have two paired organs called hemipenes. Tuataras must use their cloacae for reproduction. Due to evolutionary convergence, turtle and mammal penises have a similar structure.

Fish
In some fish, the gonopodium, andropodium, and claspers are intromittent organs (to introduce sperm into the female) developed from modified fins.

Invertebrates


In male insects, the structure analogous to a penis is known as aedeagus. The male copulatory organ of various lower invertebrate animals is often called the cirrus.

In 2010, entomologist Charles Linehard described a new genus of barkflies called Neotrogla. Species of this genus have sex-reversed genitalia: females have penis-like organs called gynosomes that are inserted into vagina-like openings of males during mating. A similar female structure has also been described in the closely related Afrotrogla. Scientists who study these insects have occasionally called the gynosome a "female penis" and insisted to drop the definition of penis as "the male copulatory organ". Motivations for using the term “female penis” include that such a term "is easier to understand and much more eye-catching" and that the gynosome have "analogous features" with male penises. Meanwhile, critics have argued that it does not fit the intromittent organ definition of "a structure that enters the female genital tract and deposits sperm".

Heraldry
Pizzles are represented in heraldry, where the adjective pizzled (or vilené ) indicates that part of an animate charge's anatomy, especially if coloured differently.