Arrow worms are hermaphrodites; male reproductive organs are in the tail, and female organs are in the trunk. They are protandric, meaning that the sperm develops first and the eggs follow later. The ovaries are present in the lower part of the trunk. Openings just before the septum divide the trunk and tail on either side of the gut. Sperm is deposited on the body and swims toward the openings, which they enter; the openings are also used for egg laying. The maximum length of mature ovaries varies with the species. The ovaries can be very long. In species such as Pterosagitta draco, when the ovary is full of eggs it can extend all the way to the neck. However, in Sagitta enflata, the ovaries are very short.
The open-ocean species are adapted to a large space and as such have not been cultured. This makes it difficult to study behavior and life cycles. For example, knowing whether they cross-fertilize or fertilize themselves would be important in studying genetic variation, but scientists have not yet determined the method of fertilization for these species. What is known about the reproductive behavior of chaetognaths comes from observations of Spadella cephaloptera. This species has been held in aquaria for observation studies. The sperm package of this species is deposited on the body of another individual of the same species. The sperm moves into the ovaries and is stored there to fertilize the eggs. The fertilized eggs are then released into the sea water. In deep-water species such as Eukrohnia fowleri, brood sacs hang from the ovary openings.
Arrow worms do not have larval stages. Small chaetognaths hatch from the eggs and then continue to grow. The animals require a temperature that is high enough to allow eggs to develop. At higher temperatures, individuals mature in a shorter period of time than they do in cool water. If the temperature is too low, they use energy in order to grow and reach a greater length. For example, the temperature of the water in which it grows greatly affects the size of Sagitta tas-manica; mature individuals range from 0.31 to 0.67 in (8-17 mm) in size. When the animals are transported to water masses that are too cold for them to reproduce, they grow to be giant in size, but they never reach a mature state and are referred to as sterile expatriates. In cold water like the Canadian Arctic, they may take two years to mature, while in tropical waters their complete life span may be as little as six weeks.
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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.