hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworm s. are elongate lindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-livinge. g, Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species Phylum Acanthocephala(棘头虫门) The thorny- headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal only three genera have been reported in human beings including Macracanthorhynchus hirudinaceusy(猪巨吻棘头虫 Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes. many of which are of medical importance. The classes main include the Class arachnida( E=) and Class Insecta(EE s!). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc MORPHOLOGY(形态学) The protozoa are small, unicellular organisms which are morphologically and fu complete. A single cell carries out all the functions such as digestion, respiration, excretion The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells Arthropods are segmented and bilaterally symmetrical (XfFk BJ) animals with a body enclosed in a stiff, chitinous(甲壳质) covering or exoskeleton(外骨骼) and bearing paired jointed appendages. The digestive system is well developed. Sexes are separate LIFE CYCLE(生活史) The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite pleted in a single host such as man. Change of host is required only to propagate the parasite in the community(eg, E histolytica溶组织阿米巴, Trichuris trichiura H s, etc). Some of the parasites require two different hosts to complete their various stage of development(e.g, Schistosoma japonicum日本血吸虫etc). In a complex life cle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e. g, Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g, Paragonimus westerman卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host.5 hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms only. Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworms, are elongate, cylindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-living(e.g., Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species. Phylum Acanthocephala(棘头虫门) The thorny-headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal, only three genera have been reported in human beings including Macracanthorhynchus hirudinaceus(猪巨吻棘头虫). Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes, many of which are of medical importance. The classes main include the Class Arachnida(蛛形纲) and Class Insecta(昆虫纲). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases. Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc. MORPHOLOGY（形态学） The protozoa are small, unicellular organisms which are morphologically and functionally complete. A single cell carries out all the functions such as digestion, respiration, excretion, reproduction, etc. The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells. Arthropods are segmented and bilaterally symmetrical（对称的）animals with a body enclosed in a stiff, chitinous（甲壳质） covering or exoskeleton（外骨骼） and bearing paired, jointed appendages. The digestive system is well developed. Sexes are separate. LIFE CYCLE（生活史） The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite are completed in a single host such as man. Change of host is required only to propagate the parasite in the community (e.g., E.histolytica 溶组织阿米巴, Trichuris trichiura 鞭虫, etc). Some of the parasites require two different hosts to complete their various stage of development (e.g., Schistosoma japonicum 日本血吸虫, etc). In a complex life cycle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e.g., Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g., Paragonimus westermani 卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host.)