Reproductive biology

Holothuroids are unique among echinoderms in having a single, anteriorly positioned gonad and gonoduct leading to a dorsal gonopore. Spawning usually is annual, occurring in the spring or summer. Some species may have a second, usually smaller, autumnal spawning event. Species can have separate sexes or be hermaphroditic. At least one deep-sea species appears to be pair forming. Among broadcast spawners, eggs and sperm are released into the water column, where fertilization and development of the larvae take place. Among brooding species, however, females gather the eggs with their tentacles as they emerge and retain them ventrally or in special pouches. A few species brood their larvae within the body cavity. Brooding is most common among littoral, cold-water species, whereas tropical taxa are almost entirely broadcast spawners, as are, apparently, many deep-sea forms. As with other echinoderms, development is largely either indirect or direct. Indirectly developing species pass through a distinct planktonic and feeding larval phase, the auricularia, before metamorphosing into a barrel-shaped doliolaria and settling as a miniature adult called a pentacula. During metamorphosis, the bilaterally arranged organ systems are reorganized into the pentamerous adult body plan. In direct development, radical metamorphosis to adult morphologic features does not occur but development proceeds directly from a nonfeeding vitellaria larva. The larva may be planktonic or not and is provisioned with lipid stores. Some tropical species may also reproduce asexually as adults by transverse binary fission.

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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