Hemichordates have two major body plans. One of the most notable characteristics of the enteropneustan hemi-chordates is their three-part body plan, which includes a pro-tosome, or anterior proboscis (sometimes called a pre-oral lobe); followed by a mesosome or collar; and finally a meta-some, or trunk. Cilia, which are present over all body areas, play roles in locomotion and in distributing the proteinaceous mucus secreted by the acorn worms. The largest species is the enteropneust Balanoglossus gigas of Brazil, an acorn worm that reaches 4.9 ft (1.5 m) in length and lives in long burrows stretching over more than 9.8 ft (3 m).
Pterobranchs have a three-part body plan like the en-teropneusts, but with a shorter, shield-shaped proboscis and a more complex collar. In some species, the collar has tenta-cled arms. Pterobranchs form colonies, often with individuals attached by so-called stolons or stems. The individual animals are called zooids and are quite small, typically less than 0.04 in (1 mm) long. Groups form and live within a co-enecium, which is a network of proteinaceous tubes built with secretions from each animal's proboscis.
The class Planctosphaeroidea has only one species, Planc-tosphaera pelagica, and it is known only from its larvae. Although several times larger at 0.3-1 in (8-25 mm) long, the almost-spherical, transparent P. pelagica larva is otherwise quite similar to enteropneust tornaria, having a gelatinous body covered with cilia. Unlike tornaria, however, the epidermis of P. pelagica has two deep invaginations (pouchlike formations) as well as numerous glands that secrete mucus.
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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.