with reptiles. It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs. Reproductive system The key anatomical difference between monotremes and other mammals gives them their name; monotreme means “single opening” in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Like reptiles, monotremes have a single cloaca. Marsupials have a separate genital tract, whereas most placental mammalian females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis while urine is excreted through the male's cloaca. The monotreme penis is similar to that of turtles and is covered by a preputial sac. Monotreme eggs are retained for some time within the mother and receive nutrients directly from her, generally hatching within 10 days after being laid — much shorter than the incubation period of sauropsid eggs. Much like newborn marsupials (and perhaps all non-placental mammals), newborn monotremes, called "puggles," are larval- and fetus-like and have relatively well-developed forelimbs that enable them to crawl around. In fact, because monotremes lack nipples, puggles crawl about more frequently than marsupial joeys in search of milk, this difference raising questions about the supposed developmental restrictions on marsupial forelimbs. Rather than through nipples, monotremes lactate from their mammary glands via openings in their skin. All five extant species show prolonged parental care of their young, with low rates of reproduction and relatively long life-spans. Monotremes are also noteworthy in their zygotic development: Most mammalian zygotes go through holoblastic cleavage, where the ovum splits into multiple, divisible daughter cells. Contrastingly, monotreme zygotes, like those of birds and reptiles, undergo meroblastic (partial) division. This means the cells at the yolk's edge have cytoplasm continuous with that of the egg, allowing the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm. Physiology Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about rather than the averages of for marsupials and for placental mammals. Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes. Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain a constant body temperature in a