![]() ![]() ![]() Bisexual flukes: (more.)įlukes have a well-developed alimentary canal with a muscular pharynx and esophagus. Hermaphroditic flukes - Clonorchis sinensis, Fasiolopsis buski, Paragonimys westermani, and Heterophytes heterpphyes. All cycles involve snails as intermediate hosts. The various stages and their unique characteristics will be reviewed in more detail as each major group of helminths is considered. With few exceptions, adult flukes, cestodes, and nematodes produce eggs that are passed in excretions or secretions of the host. All other flukes and tapeworm species that infect humans are hermaphroditic. The blood flukes and nematodes are bisexual. This lack means that nutrients must be absorbed through the tegument. Tapeworms are unique in lacking an alimentary canal. Internally, the alimentary, excretory, and reproductive systems can be identified by an experienced observer. Male nematodes of several species possess accessory sex organs that are external modifications of the cuticle. Prominent external structures of flukes and cestodes are acetabula (suckers) or bothria (false suckers). The outer covering of helminths is the cuticle or tegument. Platyhelminths and nematodes that infect humans have similar anatomic features that reflect common physiologic requirements and functions. Stages of Helminths Commonly Responsible for Pathologic Changes in Humans. The contributions of various stages to disease are listed in Table 86-2. Knowledge of the different stages in relation to their growth and development is the basis for understanding the epidemiology and pathogenesis of helminth diseases, as well as for the diagnosis and treatment of patients harboring these parasites. ![]() Table 86-1 gives the names applied to various larval helminths. Helminths develop through egg, larval (juvenile), and adult stages. This chapter deals with the structure and development of the three major groups of helminths. These groups are subdivided for convenience according to the host organ in which they reside, e.g., lung flukes, extraintestinal tapeworms, and intestinal roundworms. Roundworms are nematodes (nemato from the Greek root meaning “thread”). ![]() In medically oriented schemes the flatworms or platyhelminths (platy from the Greek root meaning “flat”) include flukes and tapeworms. The helminths are invertebrates characterized by elongated, flat or round bodies. ![]()
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