Physical characteristics

Like other platyhelminths, monogeneans are acoelomate animals; that is, they do not have a body cavity lying between the body wall and the digestive tract. Their bodies are covered by a living syncytial tegument. The digestive system consists of a pharynx, which is a muscular tube used to suck in food, and a saclike or branched intestine with no anus. The pharynx may or may not be glandular. Monogeneans range in length from about 0.04-0.08 in (1-2 mm), as in some gy-rodactylids, to more than 0.75 in (2 cm), as in some capsalids. Large monogeneans tend to be flat and leaf-shaped, but the smaller parasites are usually cylindrical. In general, these flat-worms are colorless and semitransparent. When on fish skin some may be virtually invisible to the human eye, either by virtue of their glass-like transparency or because they contain scattered pigment that matches pigment in the host's skin. These features may protect the parasites from being eaten by predatory fishes or crustaceans ("cleaner" organisms). The brown/black coloration of the polyopisthocotyleans is associated with their digestive system and derived from their blood meals.

The most significant anatomical feature of monogeneans is their possession of a posterior attachment organ or haptor armed with hooks. The hooks usually fall into two groups: small hooklets, which are often called marginal hooklets, and larger hooks called hamuli or anchors. The hooklets are essentially found in larvae although they often persist, usually without further growth, in adult monogeneans. The hooklets are specialized for attachment to the upper layer of cells (epidermal cells or Malpighian cells) in the host's skin; they fasten themselves in the web of filaments made of keratin known as the terminal web, which lies beneath the apimal membrane of the host epidermal cell. A basic and maximum number of 16 hooklets occurs in monopisthocotyleans and in polyopisthocotyleans, although they may be reduced to 14, 10, or lost altogether. In many but not in all monogeneans, the hooklets are supplemented as the parasite grows by one or two pairs of hamuli, which are usually large enough to penetrate through the epidermis (the outer layer of the host's skin) into the dermis, which is the thicker layer of skin just below the epidermis. Some monopisthocotyleans have hap-tors that become more elaborate during their development, either by acquiring glands or by subdividing into separate small compartments that function as suckers. As typical poly-opisthocotyleans grow, they develop three or four pairs of muscular suckers or clamps; each one is at the site of a hook-let. Many monopisthyocotyleans are able to move like leeches since they have suckers or glands that secrete sticky material on the lateral borders of the head.

The ovary consists of a germarium, which produces egg cells, and an extensive vitellarium that produces vitelline cells. The vitelline cells do not contain genetic material; they secret substances that form a chemically and physically resistant eggshell of tanned protein (sclerotin) and provide food for the growing embryo. Gyrodactylids are exceptional viviparous. Monogeneans are hermaphrodites; many have hard structures (sclerites) supporting the penis, while others have an eversible cirrus. There may be one or more vaginae, often with supporting sclerites, but hypodermic impregnation (through the skin) also takes place.

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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