of the epididymis. In the monotreme echidna, the proportion (26% of total) of mature sperm stored intra-abdominally in the terminal segment of the epididymis is considerably less than the proportion of mature sperm stored in the epididymis of many eutherian mammals (50-75% of total) with descended testes. Hence, both reptiles and the monotreme echidna appear to have relatively limited sperm storage capacity in the testicond epididymis compared to mammals with the epididymis located in an extra-abdominal scrotum. This reduced sperm storage capacity of the monotreme testicond epididymis is further supported by observations that the sperm storage region of the epididymis of a testicond mammal (echidna) and a scrotal mammal (rat) are respectively 4% and 8% of the total length of the duct. Significantly, the low intra-abdominal sperm storage capacity of the echidna epididymis helped inform the role of the epididymis as a prime mover in the evolution of descended testes in mammals whereby lower extra-gonadal temperatures within the scrotal cauda epididymis reduces oxidative respiration of sperm, which enhances oxygen availability, thereby allowing greater epididymal sperm storage in the cooler scrotum of mammals. Structural differentiation of the epididymis in marsupials and placental mammals Most species of marsupial (metatherian) and placental (eutherian) mammals have evolved extra-gonadal testes, although a limited number of these mammals remain testicond or exhibit differing degrees of testicular descent. As a result of the epididymis being attached to the testis, and the cauda epididymis extending below the lower extremity of the testis (Figure 1C), it was proposed that the epididymis was the prime mover in the evolution of testicular decent, whereby the cauda epididymis preceded the testis into a scrotal location. The epididymis of marsupials (metatherians) and placental mammals (eutherians) has undergone further structural differentiation compared to that observed in prototherian mammals (Figure 1). In scrotal mammals, an initial segment is nearly always observed, however, additional histologically distinct regions have developed between the initial segment and the distal sperm storage region (terminal segment). These intervening histologic regions have been referred to as the middle segment. The histologic regions of the middle segment (Figure 1C) can vary in number in metatherian and eutherian species of mammals. Beyond the histologic regions of the middle segment, the sperm storage region (anatomical cauda, histologic terminal segment) of scrotal mammals has enlarged to accommodate enhanced storage of sperm (Figure 1C). The storage of sperm in the scrotal epididymis is enhanced by cooler extra-abdominal