TREMATODA. 653 



(Ercolani), but as yet these statements have not been confirmed. A differ- 

 ence in size between specimens of D. macrostomum from the intestines of 

 Songsters and Water-rails has been noted. 



The genus Holostomum, usually classed with the digenetic Trematoda, 

 has perhaps a direct development. The sexual worm inhabits the digestive 

 tract of various carnivorous birds. The organisms known as Diplostomum 

 and Tetracotyle, which occur encysted in various Vertebrata, are believed 

 by Von Linstow and others to be the non-sexual states of this genus. The 

 ova are large v The larva of H. cornucopiae is ciliated, except anteriorly, 

 and has two eye-specks. 



Digenetic Trematoda are found entoparasitic in members of all classes 

 of Chordata. They usually inhabit various parts of the digestive tract. 

 Man may be infested by Fasciola hepatica (ramifications of bile ducts) ; by 

 Distoma lanceolatum (gall-bladder and bile ducts) ; by D. crassum 

 (duodenum) in India and China : by D. Ringeri in the lungs, causing the 

 endemic haemoptysis of Formosa ; and by several Distomata of rare occur- 

 rence, together with the formidable Bilharzia haematobia, which lives in the 

 blood-vessels of the urinary bladder, mesentery and portal system, and is 

 widely distributed over the African continent. The ova of the last-named 

 escape with the urine. Ruminants are principally infested by Fasciola 

 hepatica and by D. lanceolatum, which cause great and widespread de- 

 struction. The former is the source of the disease known in this country 

 as rot or liver-rot. Its embryo enters the small amphibious Pulmonate, 

 Limnaeus truncatulus, in which it becomes a Sporocyst. The latter in turn 

 gives origin to several generations of Rediae. The Cercaria is set free and 

 encysts itself on some foreign object, the stems of water-plants and grass, 

 which are eventually devoured by sheep, cows, &c. A very large number 

 of species of digenetic Trematoda are known which can be arranged under 

 relatively few genera. 



The Class Trematoda may be divided as follows 



i. Monogenetic Trematoda ; development direct. 



(a) Tristomeae : body roundish or elongate ; posterior extremity of body 

 never specially developed. Two adoral suckers often present; a large ventral 

 sucker often armed with chitinoid structures. Sexual apertures on the left side or 

 admedian. Laurer-Stieda canal single or double. Ova with a filament at one pole 

 only. 



Three sub-families : Tristomidae, with the single genus Tristomum Mono- 

 cotylidae, with Calicotyle, Pseudocotyle, and Monocotyle ; Udonellidae^ with the single 

 genus Udonella. 



(ti) Polystomeae : body elongate, pointed and narrow anteriorly ; broad 

 behind, and generally provided with special organs of adhesion in the shape of 

 suckers or chitinoid hooks, of suckers or claspers with chitinoid structures. Two 

 adoral suckers in some instances. Sexual apertures median. Laurer-Stieda canal 



