Intromittent organ
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An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation.[1][2] Intromittent organs are found most often in terrestrial species, as most non-mammalian aquatic species fertilize their eggs externally, although there are exceptions. For many species in the animal kingdom, the male intromittent organ is a hallmark characteristic of internal fertilization.[3]
Species with intromittent organs
Invertebrates
Molluscs
Male cephalopods have a specialized arm, the hectocotylus, which is inserted into the female's mantle cavity to deliver a spermatophore 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.[4]
Arachnids
In spiders, the intromittent organs are the male pedipalps, even though these are not primarily sexual organs, but serve as indirect mating organs; in the male the pedipalps have hollow, clubbed tips, often of complex internal anatomy. The sexually mature male typically deposits semen onto a specially woven silken mat, then sucks the emission into his pedipalps. In mating, he inserts the openings of the pedipalps in turn into the epigyne, the female external genital structure.[citation needed]
In Solifugae, sperm transfer is also indirect; the male deposits a spermatophore on the ground, picks it up in his chelicerae, then inserts it into the female's genital opening.
In Opiliones (harvestmen), males have a structure called a penis, which is not present in other arachnids.
Millipedes
In most
Insects
Male insects possess an
Vertebrates
Fish
In male members of
Members of
Tetrapods
In lizards and snakes, males possess paired hemipenes, each of which is usually grooved to allow sperm transport and spiny or rough at the tip to allow firm attachment to the female.[9] 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.[10] In reptiles, the phallus has an open sulcus instead of a closed urethral tube.[11]
In some turtles, crocodiles, some birds, and in all mammals, males possess a penis centered along the midline of the body. During copulation it becomes erect due to engorgement with blood or lymph, though in many animals it also contains a stiff or even bony support structure. When not in use, its soft penile tissue is usually flaccid, and depending on the species, may be retracted into the body. The anatomy of the penis varies widely according to species. However, the penis evolved only once in the evolutionary history of amniotes.[12] In male caecilians, the intromittent organ is called the phallodeum.[13][14]
Mammals
All male mammals have a penis. Eulipotyphlans, bats, rodents, carnivorans, and most primates (but not humans) have a bone called the baculum or os penis that permanently stiffens the penis. During copulation, blood engorges the already-stiff penis resulting in a full erection.[15]
Monotreme penes are variously unusual; the platypus has a penis with a two-lobed (bifid) tip though the whole shaft is inserted in mating, possibly to engage both of the uterine branches, but the echidna's penis actually has four heads, only two of which function at a time.[16][17] Both monotremes and marsupial moles are the only mammals with internal penises, located on the cloacal wall instead of outside of it as in other mammals.[18][19]
Most
Birds
Although birds reproduce through internal fertilization, 97% of males completely lack a functional intromittent organ.[24] For the 3% of birds with an intromittent organ, copulation occurs through brief insertion of the male organ into the vagina before ejaculation.[25] Alternatively, for the vast majority of birds—a group comprising nearly 10,000 species[26]—sperm transfer occurs by cloacal contact between the male and female, in a maneuver known as the "cloacal kiss".[25] Birds are one of the only groups which reproduce through internal fertilization but have repeatedly lost the intromittent organ.[27]
Male ostriches have a conical shaped penis that is wider at the base.[28]
A functional intromittent organ is known to be present in most species of
References
- Kardong, Kenneth V. (1995). Vertebrates: Comparative Anatomy, Function, Evolution. Dubuque, Iowa: Wm. C. Brown Publishers. pp. 567–570. ISBN 978-0-06-921991-9.
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- ^ Eberhard, W. G. (1985). Sexual selection and animal genitalia. Cambridge, MA: Harvard University Press.
- ISBN 9789401786485. Retrieved 23 August 2015.
- ^ "Reproductive behaviour in invertebrates". Encyclopædia Britannica. Retrieved 23 August 2015.
- ISBN 9780521113892. Retrieved 23 August 2015.
- ^ "Urogenital Anatomy of the Dogfish Shark". Maricopa Community Colleges. Retrieved 23 August 2015.
- ISBN 9783110106619. Retrieved 23 August 2015.
- ^ "Hemipenes of snakes and lizards". University of Colorado Boulder Museum of Natural History. Archived from the original on 26 August 2015. Retrieved 23 August 2015.
- ^ Annual Report of the Board of Regents of the Smithsonian Institution. 1900. p. 700. Retrieved 23 August 2015.
- ^ Gredler, Marissa L., et al. "Evolution of external genitalia: insights from reptilian development." Sexual Development 8.5 (2014): 311-326.
- PMID 26766229.
- ISBN 978-1-4822-8014-2.
- ISBN 978-0-8018-4780-6.
- ^ "Baculum (penile bone) in mammals". Map of Life. Retrieved 23 August 2015.
- ISBN 9780868401430. Retrieved 23 August 2015.
- PMID 18171162.
- ^ Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
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- ^ ISBN 9780199886753.
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