An intromittent organ is a general term for an external organ of a male

Male

Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 organism that is specialized to deliver sperm

Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 during copulation

Sexual intercourse

Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which a male's penis enters a female's vagina for the purposes of sexual pleasure or reproduction. The entities may be of opposite sexes, or they may be hermaphroditic, as is the case with snails...

. Intromittent organs are found most often in terrestrial species, as most aquatic species fertilize their eggs externally

External fertilization

External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....

, although there are exceptions.

Invertebrates

Male cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s have a specialized arm, the hectocotylus

Hectocotylus

A hectocotylus is one of the arms of the male of most kinds of cephalopods that is modified in various ways to effect the fertilization of the female's eggs. It is a specialized, extended muscular hydrostat used to store spermatophores, the male gametophore...

, which is inserted into the female's mantle cavity to deliver a spermatophore

Spermatophore

A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...

 during copulation. In some species, the hectocotylus breaks off inside the female's mantle cavity; in others, it can be used repeatedly to copulate with different females.

Insects

Male insects possess an aedeagus

Aedeagus

An aedeagus is a reproductive organ of male insects through which they secrete sperm from the testes during copulation with a female insect...

, whose function is directly analogous to that of the vertebrate penis. Some insects also have claspers

Valva

In biology, a clasper is a male anatomical structure found some groups of animals, used in mating.Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve as intromittent organs used to channel semen into the female's cloaca during mating...

. Male moths have an additional organ called the juxta

Juxta

For the open source bibliographical software, see Juxta .The juxta is an organ in the males of most Lepidoptera that supports the aedeagus, the organ used for reproduction in insects. The juxta is located between the two valvae....

, which supports the aedeagus.

Fish

In male members of Chondrichthyes

Chondrichthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 (shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s and rays

Batoidea

Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

), as well as now-extinct placoderms

Placodermi

Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...

, the pelvic fins bear specialized claspers

Valva

In biology, a clasper is a male anatomical structure found some groups of animals, used in mating.Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve as intromittent organs used to channel semen into the female's cloaca during mating...

. During copulation, one clasper is inserted into the female's cloaca

Cloaca

In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

, and sperm is flushed by the male's body through a groove into the female.

Members of Poeciliidae

Poeciliidae

Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...

 are small fishes that give birth to live young. In males, the anal fin is shaped into a grooved, rod-shaped organ called a gonopodium used to deliver sperm to females.

Tetrapods

In lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s and snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s, males possess paired hemipenes

Hemipenis

A hemipenis is one of a pair of intromittent organs of male squamates .Hemipenes are usually held inverted, within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. Only one is used at a time, and some evidence indicates males alternate use between...

, each of which is usually grooved to allow sperm transport and spiny or rough at the tip to allow firm attachment to the female. To become erect, a hemipenis is evaginated (turned inside out) through muscle action and engorgement with blood. Only one is inserted into the female's cloaca at a time.

In some turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, crocodile

Crocodile

A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s, some bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, and mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, males possess a penis

Penis

The penis is a biological feature of male animals including both vertebrates and invertebrates...

 centered along the midline of the body. During copulation it becomes erect due to engorgement with blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 or lymph

Lymph

Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...

. When not in use, it is usually flaccid, and depending on the species, may be retracted into the body. The anatomy of the penis varies according to species.

Male ostrich

Ostrich

The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

es have a conical shaped penis that is wider at the base. Male duck

Duck

Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

s have a penis that is coiled along the ventral wall of the cloaca when flaccid and which may have an elaborate spiral shape when erect.

All male mammals have a penis. Insectivores

Insectivora

The order Insectivora is a now-abandoned biological grouping within the class of mammals...

, bat

Bat

Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...

s, rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s, carnivorans

Carnivora

The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...

, and most primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s (but not human

Human

Humans are the only living species in the Homo genus...

s) have a bone called the baculum

Baculum

The baculum is a bone found in the penis of most mammals. It is absent in humans, but present in other primates, such as the gorilla and chimpanzee.The bone aids in sexual intercourse.-Purpose:...

or os penis that permanently stiffens the penis. During copulation, blood engorges the already-stiff penis resulting in a full erection.