Physical characteristics

Secernenteans vary greatly in size from microscopic to several feet long. The largest known secernentean, which is up to 30 ft (9 m) in length, lives in the placentas of female sperm whales. The body of secernenteans consists of a flexible cylinder that tapers at both ends, with a pointed tail and a blunt head. They are considered non-segmented pseudo-coelomates; that is, creatures possessing a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).

A flexible but tough collagenous cuticle surrounds the body with a system of grooves across the body from head to tail, which protects them internally. The non-cellular cuticle varies from four to two layers and is almost always transversely striated. Laterally for most of the body length, the cuticle is generally modified into a wing area that is marked by longitudinal ridges; generally, this region is only slightly above the normal body contour. However, in some parasitic forms, it may extend out a distance equal to the body's diameter. The cellular hypodermis is the subcuticular layer that secretes the cuticle.

The sensory system contains phasmids, which are a pair of bilateral cuticular, glandular organs, situated laterally in the caudal (posterior to the anus) region and opening to the surface by a slit or pore. Also known as precaudal glands, phas-mids are unique to the secernenteans, in which their function is believed to be sensory. At the other end are pore-like am-phid apertures, which are a pair of glandular chemosensory organs situated dorso-laterally in the cephalic (head or anterior) region and opening through the cuticle. Although usually pore-like, in isolated instances the aperture can be an oval or a cleft. The apertures show little variation throughout the secernenteans. The amphids are always labial (located on the lips). The external amphidial aperture is usually less well developed than in adenophorean worms.

Somatic and cephalic setae, which are elongated structures jointed with the cuticle, are rare. When present, the cephalic sensilla are located on the labial region, and they are porelike or papilliform. In males, there may be caudal setae. In females, somatic setae are absent. Generally, sixteen sensilla are present in the shape of two circles (an inner circle of six, and an outer circle of 10). In some parasitic groups, the number of cephalic sensilla may be reduced. Deirids, pairs of porelike sensilla that usually protrude above the surface of the cuticle, are usually present on the cervical region near the level of the nerve ring.

Secernenteans contain no hypodermal glands, but the hypodermal cells of the hypodermis (a thin tissue layer beneath the cuticle that thickens to form the dorsal, ventral, and two lateral hypodermal chords, and extends the length of the body) are usually multinucleate (syncytial: more than two nuclei), but may also be uninucleate (cellular: one nuclei). These divide the muscle cells into four fields. A layer of longitudinal muscles underlies the hypodermis.

Bursae, or caudal alae, are sublateral projections generated by a longitudinal widening of the cuticle. It is common for them to be present within male secernenteans, each looking like a fluid-filled body sac. The thin cuticle extensions are located on both sides of the anus, specifically on either side of (or sometimes surrounding) the cloaca (the urogenital opening) of males. The well-developed excretory system is in the shape of an H or U. It is a simple tubular system that is located in one or both of the lateral hypodermal chords, and embedded between the three cell bodies in the hypodermal chord. The system opens ventromedially by way of a cuticu-larized duct. The rectal part of the gland system is usually present. They have no caudal glands.

The muscular esophagus or pharynx varies in configuration, but the majority of secernenteans have three esophageal glands: two that are subventral and one that is dorsal. The subventral glands open into the posterior metacarpus. The dorsal gland opens anteriorly into the procorpus or the anterior metacarpus. Its basic structure is corpus (the anterior part is cylindrical), with the basal (bottom) region sometimes swollen in the shape of a bulb. The glands empty their contents into the esophagus to aid in digestion. The tail is the region between the anus and the back tip of the body.

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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