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460 Worms, Platyhelminthes encircling proglottid; uterus tubular or branched, with or without preformed uterine pore; vagina usually opening posterior to cirrus-sac, occasionally dorsal or ventral to cirrus-sac; common genital pore often associated with muscular lips Habitat: Adults parasitic in spiral intestine, stomach, rarely gall bladder, of elasmobranchs A bewildering variety of neural organization and complexity characterizes the platyhelminths (e.g., see Halton and Gustafsson, 1996) The basic plan involves an apical subepidermal brain consisting of multiple neurons, which can be uni, bi (especially in Neodermata), or multilobed This is a true brain because it controls reflexes in the peripheral nerve net The number of nerve cells (neurons) in the brain varies between 50 and 550 in free-living species, but this number is less well known for the neodermatans The brain is generally connected to two, or sometimes more, main longitudinal nerve cords, each consisting of axons with cell bodies distributed at irregular intervals along the length of the cords These main nerve cords are usually connected to one another by numerous transverse commissures in a ladder-like arrangement In addition, a peripheral array of nerve plexuses (or nerve-nets) is found throughout the body The attachment organs and the components of the female reproductive system involved in egg formation are especially well supplied with nerve plexuses, but plexuses are also found associated with the pharynx and intestine and below the surface and below the muscle layers of the body There are separate plexuses for sensory, integrative and motor activities The central nervous system (brain plus main cords) is more emphasized in neodermatan groups than in non-neodermatan groups Cestodes may exhibit regional differences in the complexity of the subtegumental plexuses along the length of the body Transmission electron microscopy suggests that platyhelminths exhibit an extensive diversity of types of sensory receptors Several different neuronal cell types are present, including unipolar, bipolar, and multipolar cells Although unipolar neurons are generally confined to the ganglia of the brain and longitudinal nerve cords, cells of plexuses are dominated by bipolar cells Individual neurons are highly secretory and contain a variety of types of vesicles in their cytoplasm Platyhelminths utilize a diversity of signaling molecules and at least two different groups of neuropeptides The nervous systems of platyhelminths appear to possess compounds similar to all of the major neurotransmitters known in taxa as complex as vertebrates In addition, there are two groups of neuropeptides that appear to be unique to flatworms: neuropeptide F and FMRPamide-related peptides (FaRPs) both of which are fairly extensively distributed among the various platyhelminth groups The existence of glial or gliallike cells in platyhelminths remains somewhat controversial foregut and a caecum The foregut leads from the mouth to the blind-ended sac-like caecum that is lined with a thin gastrodermis Free-living species tend to be microphagous or predatory and feed on bacteria, unicellular alga, protists, and almost all types of invertebrates Characteristically, the freeliving species have a highly muscular suctorial pharynx Depending on the species, the pharynx can be protruded, inserted, applied to, extended over, or used to envelop and swallow whole the various foodstuffs they actively seek or encounter Food passes from the mouth via the pharynx to the esophagus and then to the caecum The gastrodermis has glandular and phagocytic cells that digest and incorporate the nutrients A number of free-living platyhelminths (e.g., prolecithophorans, rhabdocoels, polyclads) harbor endosymbiotic algae that serve as an additional source of food The solid waste products of free-living species are often released via a genitointestinal canal, rather than a separate anus; in some groups (e.g., polyclads), anal pores assist with this function Symbiotic and parasitic platyhelminths vary their feeding habits according to the hosts on, or in, which they live Groups that possess suckers may have numerous secretory cells and sensory structures surrounding the mouth and sucker Although similar in basic design to the foregut of freeliving groups, the parasitic groups tend to exhibit distinct cell type configurations in the gastrodermis In these groups, the junction between the foregut and caecum is invariably abrupt Aspidogastreans have a single cell type in their gastrodermis that alternates cyclically between a secretory–absorptive phase and an autophagic–exocytotic phase Digeneans have a syncytial or cellular gastrodermis with only one cell type, and digestion takes place extracellularly in the lumen of the caecum There is evidence, however, that some digeneans are capable of acquiring nutrients directly through microvillar extensions of their body surfaces In contrast, digestion in the caecum of monogeneans is intracellular The polyopisthocotylean monogeneans are blood-feeders The nonpigmented cells in these taxa are thought to support and protect the pigmented digestive cells in the gastrodermis In addition, at least some of these taxa possess a buccoesophageal canal between the buccal cavity and the esophagus that permits regurgitation of intestinal contents The monopisthocotyleans feed on the epidermal cells and mucus of their host and have only a single gastrodermal cell type involved with endocytosis and digestion In the Cestoda and some symbiotic fecampiids all vestiges of a digestive system have been lost These species rely solely on acquiring nutrients through their outer body wall Among the Cestoda, and indeed all other neodermatans, the neodermis plays an active role in the biochemical transport of nutrients In cestodes the surface area of the neodermis is markedly increased by the presence of microtriches, which enhance the availability of readily available nutrients to be actively absorbed into the parasite’s body Feeding and Digestion Excretion and Osmoregulation Other than a few fecampiids, dalyelloids, and the cestodes, flatworms generally have a digestive system divisible into a The excretory system of platyhelminths is thought to function primarily in osmoregulation, with the removal of metabolic Nervous System
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