The penis morphology of the four-eyed opossum Philander opossum.

The penis of the Philander opossum is found in post-scrotal position and attached to the schiatic arch by means of a suspensory ligament. The bulk of the penile musculature is formed by the ischio-cavernosus (IC) and bulbo-spongiosus (BS) muscles, which converge caudo-medially to join the radix penis. The paired levator penis muscles are encaised in the radix and fuse to each other ventrally to the urethra forming a long tendon. The erectile tissue arisen from the BS forms the corpus spongiosum of the urethra and the corpus cavernosum starts within the IC. The urethral mucosa is longitudinally folded and lined with transitional epithelium. The body of the penis shows a pronounced sigmoid flexure which runs ventro-dorsally, with ventral, intermediate and dorsal segments. The retractor penis (RP) muscles insert in both sides of the penile dorsal curvature. The fascia of the penis is thick and surrounds the corpus cavernosum, the corpus spongiosum of the urethra and the final part of the levator penis muscles. The body-glans transition initiates with the formation of a mid- ventral groove which progressively reaches the urethral lumen; the mid-dorsal groove initiates farther distally and also reaches the urethra. The transition of urethra into urethral grooves is characterized by the modification of transitional into stratified squamous epithelium. The glans penis is bifid and has slender tips. Each urethral groove are situated on the medial-ventral surface of each glands tip. The penis in the non-erect state is partially situated in the preputial cavity; when erected, projects through the cloaca, unfolding the sigmoid flexure. The preputial sac is formed by the lamina externa (without hair follicles but having preputial glands), the lamina interna (which shows cornified areas and delimit externally the opening of the preputial cavity) and the visceral lamina, very folded and with numerous cornified spicules.