Eggs and sperm are released into the water. How spawning is synchronized is not known. In subtropical populations the reproductive period stretches over several months and individuals spawn repeatedly. In temperate areas the reproductive period is shorter.
Total, radial cleavage is followed by the formation of a blastula, then gastrulation and neurulation ensue, and a peculiarly asymmetric larva is formed. The large mouth is situated on the left side. A single row of gill slits forms and the larvae start feeding immediately. More gill slits are added and later an additional row of gill slits forms on the right side. When about 12-15 pairs of gill slits are formed the larvae sink to the bottom and metamorphose into juveniles. This metamorphosis mainly involves the formation of the atrium by an outgrowth of the epidermis that surrounds the pharynx. Simultaneously the gill slits are divided and new gill slits are added. From now on the animals only grow in size, adding additional gill slits and segments while developing the gonads to full maturity. The larval period in the plankton lasts from several weeks up to a few months. In the following year the animals become sexually mature. Offshore plankton samples yielded relatively large cephalochordates that resembled larval stages but had developed rudimentary gonads. These were described under the genus name Amphioxides. However, it is more likely that these forms are merely larval forms of typically benthic species that did not encounter a suitable substrate rather than representing a genus with a different life history.
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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.