Reproductive biology

Enteropneusts normally reproduce sexually via external fertilization, and develop either directly or via tornaria larvae. The indirect developers, including Balanoglossus and Pty-chodera species, are in the majority. These species develop from egg to planktonic tornaria larva to adult form. The tornaria larvae eventually become sessile, with the burrow-dwellers developing tails behind the anus that they use to anchor themselves in their mucus-lined tunnels. Direct developers, on the other hand, hatch into adult animals, bypassing the planktonic phase. An example is Saccoglossus kowalevskii. Enteropneusts are also known to reproduce asex-ually by fragmentation of the adult's body, but this mode of reproduction is uncommon. Typically, the females lay up to 3,000 eggs at a time, and the males release sperm that appear to find the eggs by following chemical cues. Reproduction in many species is cyclical. Saccoglossus horsti, for example, breeds in late spring to midsummer. Water temperature and tides appear to affect reproductive timing in hemichordates.

The pterobranchs reproduce via short-lived larvae in a fashion similar to the enteropneusts, but more often resort to reproduction by asexual budding. Many, perhaps all, of the hemichordates are able to regenerate portions of their trunks.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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