TREMATODES (flukes), a class of Platyhelminthes (q.v.) in which the body is unsegmented and without a cellular epidermis or external cilia, and an alimentary canal is present. All the mem bers of the group are parasitic.
General Morphology.—The body is generally flattened and leaf-like or ribbon-like in shape, but may be relatively stout and oval or circular in transverse section. One or more muscular suckers are usually present on the ventral surface. In general structure the Trematodes closely resemble the free-living Tur bellaria (q.v.). They differ from them in the development of suckers and other organs for attachment to the host, and in the absence (in the adult) of a ciliated epidermis.
The external covering is a stout cuticle, often armed with spines, below which is a subcuticular layer containing unicellular glands. The number and arrangement of the suckers are very variable. Usually there is an anterior sucker or pair of suckers, and a posterior sucker, or a posterior disc-like organ of attach ment provided with subsidiary suckers or with Chitinoid hooks of various kinds. The complex development of the posterior organ of attachment is characteristic of the ectoparasitic forms (Monogenea). In endoparasitic forms (Digenea) the posterior sucker is usually a simple muscular disc or cup, and may be situ ated at the posterior end of the body, but is more often displaced anteriorly so as to lie in front of the middle or even close behind the oral sucker.
Pigment is rare in Trematodes, but occurs in the parenchyma of certain species, more especially among the Monogenea. The body of endoparasitic species may, however, appear more or less brightly coloured on account of the contents of the intestine and uterus, and the vitelline glands, showing by transparency.
The musculature usually consists of an outer circular and an inner longitudinal layer of fibres. Oblique and dorso-ventral fibres are also frequently present.
Except in one family, the mouth is situated at or near the anterior extremity. It may open through an anterior, or oral, sucker, or, in certain ectoparasitic forms, may be flanked by a pair of adoral suckers. The alimentary canal may be a simple, blind sac, but usually consists of a relatively short median an terior portion and two posterior branches arising by the bifurca tion of the former. These branches may be simple or secondarily branched, and may join again posteriorly, or may remain distinct.
In the great majority of forms they are without any posterior opening, but in one or two species they have been found to open to the exterior by one or a pair of pores. The median anterior portion of the gut may be differentiated into a muscular sucking bulb, or pharynx, and a simple tubular portion between this and the bifurcation, called the oesophagus.
The whole of the space between the wall of the alimentary canal and the external cuticle is filled up with a spongy connective tissue, or parenchyma, in which the other organs are embedded, as in other classes of Platyhelminthes. The nervous system con sists of a pair of central ganglionic masses (the "brain"), situ ated anteriorly and dorsally, and longitudinal nerve-cords (of which there may be as many as four pairs) running posteriorly throughout the body, and connected at intervals by transverse commissures. Eyes are present in some of the ectoparasitic forms, and in the larval stages of some endoparasitic species, but are absent in the adults of the latter. There may also be "tactile cones" on the surface of the cuticle, consisting of small eleva tions surmounted by groups of stiff cilia.
The excretory system is composed, as in other Platyhelminthes, of branching canals whose smallest branches end in flame-cells.
There are usually two main longitudinal canals, which may open independently, through a pair of excretory sacs, on to the dorsal surface, or may lead into a common posterior "bladder." With few exceptions, the Trematodes are hermaphrodite. The complex system of genital organs is arranged on the same general plan as in other groups of Platyhelminthes, but is subject to much variation in details, and is of great importance from the taxonomic standpoint. The testes are almost invariably paired, and are usually compact organs, though they may be deeply lobate or even elaborately branched. The male ducts unite and open through a muscular intromittent organ, or cirrus, which is usually contained in a special pouch (the cirrus-sac). The ovary is single, and, like the testes, may be compact or more or less branched. The vitellaria generally consist of two lateral series of follicles, those of each side being connected by branching ducts with a main yolk-duct, and the two main ducts crossing the body to join in the median region, near the shell-gland.