The female ovary contains germ cells that give rise to eggs. Fertilization, most of the time by male sperm, takes place in the uterus and eggs are released through the vagina. Males have one testis (monarchic), when testes are present. Most species produce males and females (that is, dioecious), but some species only produce females, in which both male and female structures are contained in the same individual. In those few cases the species are hermaphroditic. Most eggs are about the same size and shape. Males produce sperm in the testes, which are shaped similar to female ovaries. Sperm accumulate in the seminal vesicle and exit through the anus. Males have accessory copulatory organs, called spicules. During mating, a spicule becomes rigid so it can be inserted into the vagina in order to form a passageway for the sperm. Male bursae, usually numbering four, are used to clasp the female during copulation. Males possess paired preanal supplement glands (or genital papillae) in two sublateral rows on the ventral side of the body. The glands are used for secretion and attachment, and function during copulation.
The life cycle is generally a straight cycle going from fertilized egg, through four juvenile (often called larval) stages involving a set number of cell divisions, and into adulthood. Secernenteans generally produce offspring through internal sexual reproduction. Fertilization occurs in the female's uterus, where the zygote is then placed inside a tough shell. Development from egg to adult is generally similar for all species, although many differences exist to the norm (often times brought about by environmental conditions and special types of life, as two examples). Usually a young juvenile hatches from the egg. It usually resembles the adult except in size and in the maturity of its sex organs. Each of the four larval stages (usually referred to as L1, L2, L3, and L4) is separated from one another by the complete shedding of the outer layer (what is called ecdysis, or molting in other animals), including the lining of the mouth and rectum. There is no increase in the number of cells after hatching, with growth coming exclusively from an increase in the size of the original cells. Development in successive stages is gradual over all, however organs themselves can develop at rapid rates.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.