Reproductive biology

All ascidians are hermaphroditic. All colonial species of the suborder Aplousobranchia are viviparous; in these species ova are fertilized internally and developed within the parent colony, in the atrial cavity of zooids or in special outgrowths of the zooid body wall, termed brood pooches, or in the common test of the colony. Viviparous species release swimming larvae. Solitary species are either viviparous or oviparous, releasing ova. In viviparous solitary species, larvae are incubated in the atrial cavity. Ascidian larvae are called tadpole larvae. Larvae have an oval trunk, usually less than 0.04 in (1 mm) long, and a tail that is longer than the trunk. The larval trunk contains the larval organs, including the cerebral vesicle with a light receptor and statocyte and adhesive papillae on the anterior end, and a rudiment of the future adult. In many colonial species this rudimentary ascidian within the larval trunk is rather well developed and resembles a zooid of the adult ascidian. Sometimes, as in colonial Diplosoma, the larval trunk may contain two or more future zooids. Ascidian larvae never feed; they swim for a short time and then attach to the substratum and start metamorphosis. During metamorphosis all larval organs degenerate, and an ascidian develops from the rudiment located in the larval trunk. In the case of solitary ascidians only one specimen develops from each larva, whereas in colonial ascidians individuals (zooids) replicate themselves by asexual reproduction (budding) and form a colony.

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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