Physical characteristics

Gastrotrichs are aquatic, strap-shaped to tenpin-shaped worms, 0.002-0.14 in (0.05-3.5 mm) long. The body is flat ventrally and arched dorsally. A multilayered, translucent cu ticle covers the entire body. The ventral epidermis is ciliated; the cilia are covered with a thin layer of epicuticle. Epidermal cells may be monociliated or multiciliated. The body generally is divided into head and trunk regions. The head bears a terminal mouth, anterior myoepithelial pharynx, and sometimes eyes or tentacles or both. The trunk contains a straight tubular intestine, at least one pair of protonephridia, reproductive organs, and a ventral anus. There is no body cavity. Cuticular duo-gland adhesive tubes may occur on the head or trunk. Muscles are present in circular, longitudinal, and helical orientations; they may be cross-striated, obliquely striated, or, rarely, smooth.

The order Macrodasyida contains strap-shaped animals, 0.006-0.14 in (0.15-3.5 mm) long. The pharynx has an inverted Y-shaped lumen and pores connecting it to the outside. Pharyngeal pores are absent in Lepidodasys. The ventral epidermal cells may be monociliated or multiciliated. Epidermal glands generally are present. Adhesive tubes often are numerous and occur anteriorly behind the mouth and posteriorly; adhesive tubes also may be present in lateral, dorso-lateral, and ventral positions. The cuticle is smooth in most species, except for the species of Thaumastodermatidae and a few others, where the cuticle forms scales, spines, or hooks. Macrodasyida are simultaneous or sequential hermaphrodites with complex male and female reproductive organs.

The order Chaetonotida contains vermiform and tenpin-shaped animals, 0.002-0.04 in (0.05-0.9 mm) long. The pharynx has a Y-shaped lumen and no pharyngeal pores. There is a pharyngeal plug at the junction between the pharynx and the intestine. Adhesive tubes typically are present only on the posterior caudal furca. Some species lack adhesive tubes (e.g., Dasydytes), while species of Neodasys (suborder Multitubulatina) possess papilla-like lateral adhesive tubes. The cuticle often

Musellifer delamarei is a rare Mediterranean species inhabiting coarse, organogenous sediment. In contrast with the vast majority of chaetonoti-dans, which reproduce by parthenogenesis, M. delamarei is a hermaphroditic species and reproduces by internal fertilization. (Photo by M. Antonio Todaro. Reproduced by permission.)

Musellifer delamarei is a rare Mediterranean species inhabiting coarse, organogenous sediment. In contrast with the vast majority of chaetonoti-dans, which reproduce by parthenogenesis, M. delamarei is a hermaphroditic species and reproduces by internal fertilization. (Photo by M. Antonio Todaro. Reproduced by permission.)

bears scales or spines or both, except in species of Neodasys and the Proichthydidae. The epidermis is monociliated in Multi-tubulatina and multiciliated in Paucitubulatina. Cross-striated muscles occur in Neodasys, and obliquely striated muscles are seen in all other species. Chaetonotida are hermaphroditic; several species of Paucitubulatina also are parthenogenetic. An anomalous reproductive organ, the X-organ, usually is present in Paucitubulatina.

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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