TREMATODA, or FLUKES, a promi nent class of the branch or phylum Plathel minthes of consistently parasitic habit. The simple body, the presence of an alimentary canal and even of some special sense organs together with the occurrence in most cases of free living stages in development shows that the trematodes stand much nearer free living forms like the Turbellaria than do the cestodes. Furthermore, while the majority of flukes are endoparasitic there are also ectoparasitic species which display considerable freedom and not only pass from host to host but have an ex tended period of free existence before they as sume the parasitic life.
Trematoda arc generally flattened and elongate though some are conical, cylindrical or irregular in form. The mouth is mostly at or near the anterior tip of the body and the excretory pore near the posterior end. Highly developed muscular suckers are found at the anterior end on the ventral surface or at the posterior end. The ectoparasitic species have commonly chitinous hooks or anchors in con nection with the suckers to serve as additional organs of attachment. Many species are more or less covered with minute scales or spines. Trematodes vary in size from 02 mm. to 25 or rarely 75 mm. in length. The body is solid as parenchymatous tissue fills all the space be tween the organs. The alimentary system is either rodlike (rhabdocoel) or shaped like a tuning fork (triclad). In some of the larger forms it possesses lateral branches which may assume a complex aspect. There is no skeletal system but muscular layers of circular, longi tudinal and diagonal fibres lying near the surface of the body form the main part of the dermomuscular sac. By virtue of this excessive muscular de velopment the flukes are ex tremely mobile and variable in fcrm. Stellate flame cells ter minating in minute tubules con stitute the elements of the ex cretory system which varies in complexity with the size of the organism.
The reproductive system comprises with rare exceptions the organs of both sexes. The male organs are rather simple, whereas the female system is very complex. The parts and their relations are very similar to those found in cestodes.
The development of trematodes is direct and simple among the ectoparasitic forms but com plex among endoparasites where it is complicated by al ternation of generations and one or more changes of host.
In the common sheep liver fluke which presents a life cycle of moderate complexity, a minute ciliated embryo es capes from the egg shell whenever this is carried by chance into a water body. The lit tle free swimming larva (miracidium) seeks out a snail and in its liver tissue metamorphoses into a sac (sporocyst) in which is developed a new generation (redia). Within these indi
viduals are produced similar forms or when conditions are favorable a modified type (cercaria) which deserts the snail and encysts on grass or in the water. This form is really the young distome and it attains the stomach of the sheep with food or drink. There set free by digestion, it wanders into the liver and by growth becomes adult. Even more complex conditions are found in other species; the most if not all endoparasitic trematodes agree in selecting a mollusk as an intermediate host and in manifesting alternation of generations.
Several species are of great economic im portance; thus the sheep liver fluke (Fasciola hepatica) produces epidemics of liver rot of great magnitude, entailing large annual losses to sheep raisers in England, Australia and on the Continent ; the loss in extreme instances has reached $20,000,000 in a single year.
About a dozen species of fluke are known as human parasites. The Asiatic lung fluke (Paragonimus westermani) is very abundant in the East (Japan, China, Formosa and the Philippines) and produces a disease resembling tuberculosis. A related species has been re ported from a few localities in the United States. The Chinese intestinal fluke (Fasciolopsis buski) though formerly believed to be rare, is now known to be exceedingly abundant in some parts of China, Siam, etc. It gives rise to serious intestinal troubles.
The human liver fluke (Clonorchis sinensis) is very abundant in some parts of China and Japan and has frequently been introduced into this country. The human blood fluke has three species; Schistosoma hwmatobium which has been identified in Egyptian mummies of the 20th dynasty (1250 a.c.) and is exceedingly common in that and adjacent regions to-day. Sch. japonicum, endemic in parts of Japan, China and the Philippines; Sch. mansoni, re ported first from the West Indies, and known to occur in Central and South America and in the Kongo Basin, Africa. It was very likely in troduced into this continent by the slave trade. All species are the cause of serious circulatory, renal and intestinal disturbances. The eggs are evacuated either with urine or feces and are distinguished in that the Egyptian form possesses a terminal spine, the Japanese form is spineless and the form from the West Indies has a lateral spine on the egg. The disease produced by the first species has long been known as Egyptian hamaturia. For further data consult Fautham, Stephens and Theobald, 'Animal Parasites of Man' (London 1916) ; Ward, H. B. 'Trematoda in Reference Hand book of Medical cal Sciences' (New York 1917).