368 MAMMALIA [REPRODUCTIVE in the large majority of the class there is a projecting fold of skin, strengthened by fibro-cartilages, called the pinna or auricle, or "external ear," of very variable size and shape, generally movably articulated on the skull, and provided with muscles to vary its position, as it helps to collect and direct the vibrations of sound into the meatus. REPRODUCTIVE ORGANS. Testes. Iu the male the testes retain nearly their primitive or internal position throughout life in the Monotremata, Sirenia, Cetacea, most Edentata, Hyracoidea, Probosddea, and Seals, but in other orders they either periodically (as in Rodentia, Insedivora, and Chiroptera} or perman ently pass out of the abdominal cavity through the inguinal canal, forming a projection beneath the skin of the perineum, or becoming suspended in a distinct pouch of the integument called scrotum. All the Marsupials have a pedunculated scrotum, the position of which differs from that of other mammals, being in front of, instead of behind, the preputial orifice. The presence, absence, or comparative size and number of the accessory generative glands prostate, vesicular, and Cowper s glands, as they are called vary much in different groups of mammals. Penis. The penis is almost always completely developed, consist ing of two corpora cavernosa attached to the ischial bones, and of a median corpus spongiosum enclosing the urethra, and forming the glans at the distal portion of the organ. In Marsupials, Monotremes, and the Sloths and Anteaters, the corpora cavernosa are not attached directly to the ischia, and in the last-named the penis is otherwise of a very rudimentary character, the corpus spongiosum not being present. In many Marsupials the glans penis is bifurcated. In most Primates, Carnivora, Rodentia, Insectivora, and Chiroptera, but in no other orders, an os penis is present. Ovaries. In the female, the ovaries retain permanently their and original abdominal position, or only descend a short oviduct. <ji s t ance ni to the pelvis. They are of comparatively smaller size than in other vertebrates, and have a definite flattened oval form, and are enclosed in a more or less firm " tunica albugenia." The oviduct has a trumpet-like, and usually fimbriated abdominal aperture, and is more or less differentiated into three portions : (1) a contracted upper part, called in Man and the higher mammals " Fallopian tube"; (2) an expanded part with muscular walls, in which the ovum undergoes the changes by which it is developed into the foetus, called the "uterus"; (3) a canal, the " vagina," separated from the last by a valvular aperture, and terminating in the urino-genital canal, or common urinal and genital passage, which in higher mammals is so short as scarcely to be distinct from the last, The complete dis tinction of the oviducts of the two sides throughout their whole length, found in all lower vertebrates, only occurs in this class in Monotremes, a prevailing mammalian characteristic being their more or less perfect coalescence in the middle line to form a single median canal. In the Marsupials this union only includes the lower part of the vagina; in most Placentalia it extends to the whole vagina and a certain portion of the uterus, which cavity is then described as "bicornuate." In the higher mammals, as in Man, and also in some of the Edentates, the whole of the uterus is single, the contracted upper portion of the oviducts or Fallopian tubes, as they are then called, enter ing its upper lateral angles by small apertures. In certain lower forms of mammals the urino-genital canal opens with the termination of the rectum into a common cloaca, as in other vertebrates; but it is characteristic of the majority of the class that the two orifices are more or less distinct externally. Mammary glands, which secrete the milk by which Mam- the young are nourished during the first portion of their mar y xistence after birth, are present in both sexes in all lancls mammals, though usually only functional in the female. In the Monotremes alone, their orifices are mere scattered pores in the skin, but in all others they are situated upon the end of conical elevations, called mammillae or teats, which, taken into the mouth of the young animal, facilitate the process of sucking. These are always placed in pairs upon some part of the ventral surface of the body, but varying greatly in number and position in different groups. In the Cetacea, where the prolonged action of sucking would be incompatible with their subaqueous life, the ducts of the glands are dilated into large reservoirs from which the contents are injected into the mouth of the young animal by the action of a compressor muscle. Secondary sexual characters, or modifications of structure Secon< peculiar to one sex, but not directly related to the repro- ai T ductive function, are very general in mammals. They ^_ a almost always consist of the acquisition or perfection of ac t e rs. some character by the male as it attains maturity, which is not found in the female or the young in either sex. In a large number of cases these clearly relate to the combats in which the males of many species engage for the posses sion of the females during the breeding season ; others are apparently ornamental, and of many it is still difficult to apprehend the meaning. Many suggestions on this subject will, however, be found in the chapters devoted to it in Darwin s work on The Descent of Man and Selection in Relation to Sex, where most of the best-known instances are collected. Superiority of size and strength in the male of many species is a well-marked secondary sexual character related to the purpose indicated above, being probably perpetuated by the survivors or victors in combats transmitting to their descendants those qualities v.hieh gave them advantages over others of their kind. To the same category belong the great development of the canine teeth of the males of many species which do not use these organs in procuring their food, as the Apes, Swine, Musk and some other Deer, the tusk of the male Narwhal, the antlers of Deer, which are present in most cases only in the males, and the usual superiority in size and strength of the horns of the Bovidss. Other secondary sexual characters, the use of which is not so obvious, or which may only relate to ornament, are the presence of masses or tufts of long hair on different parts of the body, as the mane of the male Lion and Bison, the beards of some Rumi nants and Bats (as Taphozous melanopogon), Monkeys, and of Man, and all the variations of coloration in the sexes, in which, as a general rule, the adult male is darker and more vividly coloured than the female. Here may also be men tioned the presence or the greater development of odori ferous glands in the male, as in the Musk Deer, and the remarkable perforated spur with its gland and duct, so like the poison-tooth of the venomous serpents, found in the males of both Ornithorhynchus and Echidna, the use of which is at present unknown. Placenta. The development of the mammalian ovum, Placei and the changes which the various tissues and organs of the body undergo in the process of growth, are too intricate subjects to be explained without entering into details incompatible with the limits of this article, especially as they scarcely differ, excepting in their later stages, from those of other vertebrates, upon which, owing to the greater facilities these present for examination and study, the subject has been more fully worked out. There are, however, some points which require notice, as peculiar to the mammalian class, and as affording at least some hints upon the difficult subject of the affinities and classification
of the members of the group.