The penis is very large, cylindrical, with a truncated, expanded, flattened termination.
The penis, and its much reduced vestige of the female, is developed from the ventral wall of the proctodaeum.
Insemination takes place either by the introduction of the penis into the cloaca of the female, or by the puncture of the bodywall of the female by the penis, and the injection of the sperm into the body cavity, whence the single spermatozoa must make their way to the eggs.
The skin of the penis forms a fold which covers the glans and is known as the prepuce or foreskin; when this is drawn back a median fold, the frenuluni praeputii, is seen running to just below the meatus.
From this an equally slender tube proceeds, which joins its fellow of the opposite side, and the two form a thick, walled tube, which opens on to the exterior within the bursa copulatrix through which the penis protrudes.
The male urethra begins at the bladder and runs through the prostate and perineum to the penis, which it traverses as far as the tip. It is divided into a prostatic, membranous and spongy part, and is altogether about 8 inches in length.
The spongy urethra is that part which is enclosed in the penis after piercing the anterior layer of the triangular ligament.
In the soft parts the caecum is very large, the penis is armed with a pair of barbed horny claspers and the scrotum is spiny.
Each testis communicates by means of an efferent duct with a common collecting duct of its side of the body, which opens on to the exterior by means of a protrusible penis, and to which is sometimes appended a seminal vesicle.
The penis is the intromittent organ of generation, and is made up of three cylinders of erectile tissue, covered by skin and subcutaneous tissue without fat.
In most species the penis (which is generally provided with a bone) may be more or less completely retracted within the fold of integument surrounding the vent, and lie curved backwards upon itself under cover of the integument, or it may be carried forward some distance in front of the anal orifice, from which, as in voles and marmots,.
The latter unite and end in a penis which opens posteriorly.
The " brush-formed " organs of the Podocopa are medially placed, and, in spite of their sometimes forward situation, Miller believes among other possibilities that they and the penis in the Cypridinidae may be alike remnants of a third pair of legs, not homologous with the penis of other Ostracoda (Podocopa included).
In the male there are a right and a left protrusible penis in every genital segment, into which opens the nephridium and a sperm-sac. The wide funnels of the nephridia of this region are possibly of coelomic origin.