1. Introduction
2. Clonorchis sinensis
3. Fasciolopsis buski
4. Paragonimus westermani
5. Schistosoma japonicum

I. INTRODUCTION

Trematodes are members of phylum platyhelminthes (扁形动物门), which also includes the cestodes or tapeworms. The parasites are known as flukes. All trematodes parasitic to humans belong to Class Trematoda, subclass Digenea (复殖亚纲).
The digenetic (世代交替的) trematodes or flukes, are among the most common and abundant of parasitic worms, second only to nematodes in their distribution. They are parasites of all classes of vertebrates, especially marine fish, and some species, as adults or juveniles, inhabit nearly every organ of the vertebrate body. Their development occurs in at least two hosts, the first a mollusk (软体动物) or, very rarely, an annelid (环节动物). Many species include a second and even a third intermediate host in their life cycles. Several species cause economic losses to society through infections of domestic animals, and others are medically important. These medical trematodes include below species.

SPECIES
Digentic trematodes constitute one of the largest groups of platyhelminths (扁形动物), parasitizing a wide range of invertebrate and vertebrate hosts.
Table II-1-1 Common parasitic trematodes

Within human hosts, these worms are found in numerous organs, including the intestine, lungs, liver and vascular system.

LIFE CYCLE
Typical Life Cycle include sexual generation (有性生殖) and asexual generation (无性生殖) (Fig II-I-1).

                 Fig II-I-1     An example of trematode life cycle

MORPHOLOGY
Despite superficial differences, the morphology of the various groups of digenetic trematodes is basically uniform. They are characteristically flat, leaf like, hermaphrodite (雌雄同体) organisms except for the schistosomes which have a boat shaped male and a cylindrical female. One or more muscular suckers (吸盘) are always present on the ventral surface (usually possess a powerful oral sucker that surrounds the mouth), and most also have a midventral acetabulum (腹吸盘) or ventral sucker (腹吸盘). Reproductive system is highly developed. Excretory and nervous are present.
The eggs of trematodes are operculated (有盖的) except for schistosomes.
Tegument (体壁组织): The tegument of the adult worm is now recognized as a dynamic, cellular structure. Under light microscopy, it appears as a generally homogenous layer about 7-16μm thick. The tegument is a syncytium(合胞体)，that is, a multinucleated tissue with no cell boundaries. The outer zone of this syncytium is external plasma membrane, a surface coat, or glycocalyx (糖萼), that varies in thickness according to species. Its function includes absorption of exogenous molecules, shielding (protection) from hostile environmental influences and sense. Surface invaginations, the number and extent of which also vary according to species, serve to increase tegumental surface area, much like microvilli (微绒毛) on the surface of human intestinal cells.  
Digestive system: It consists of mouth (oral sucker), prepharynx, pharynx, esophagus and intestine which is divided into two cecums (肠管).
Reproductive system: Most trematodes are hermaphroditic (雌雄同体的) except schistosoma. Male reproductive system consists of testis, vas efferens (输出管)，vas deferens (输精管), seminal vesicale (储精囊), prostatic gland (前列腺), ejaculatory duct (射精管) or cirrus (阴茎), and cirrus pouch (阴茎囊) etc. Female reproductive system consists of ovary (卵巢), oviduct (输卵管), ootype (卵模), Mehlis’ gland (梅氏腺), seminal receptacle (受精囊), Laurer’s canal (劳氏管), vitellaria (卵黄腺), vitelline duct (卵黄管), common vitelline duct (总卵黄管), vitellaria reservoir (卵黄囊), uterus (子宫) and metraterm (子宫末端) etc.
Excretory system  It consists of flame cells (焰细胞)，collecting tubes (集合管) and excretory bladder (排泄囊). These flame cells provide the basis for the identification of the species. (Fig Ⅲ-Ⅰ-2)