TREMATODA. 163 



the final larvae. Each redia is a cylindrical organism with a 

 short alimentary canal (Fig. 72 (6)). 



Like the sporocysts, the rediae give rise internally to more 

 embryos, of which some are simply rediae over again, while 

 the last set are quite different, — long-tailed cercaria, with 

 two suckers and a forked food canal. These emerge from 

 the rediae, wriggle out of the snail, pass into the water, and 

 moor themselves to stems of damp grass. There they lose 

 their tails and become encysted. If the encysted cercaria 

 on the grass stem be eaten by a sheep, it grows, in about 

 six weeks, into the adult sexual fluke. 



It will be noted that the sporocyst is the modified embryo, but that it 

 has the power of giving rise asexually to rediae. These develop, how- 

 ever, from special cells of the sporocyst, which we may compare to 

 spores or to precociously developed parthenogenetic ova. Though the 

 reproduction is asexual, it is not comparable to budding or division. 

 The same power is possessed by the rediae, and jthere are thus several 

 (at least two) asexual generations between the embryo and the adult. 



The disease of livei rot in sheep is common and disastrous. It has 

 been known to destroy a million sheep in one year in Britain alone ; and 

 in the winter 1879-80 the mortality attributed to fluke disease was 

 estimated at three millions. It is especially common after wet seasons, 

 and in damp districts. 



Classification. — Trematodes may be conveniently arranged, 

 though not exactly classified, according to their mode of development. 



A. Trematodes with direct development — Monogenetic. 



e.g. Polystomum integerrimum. This form with many suckers 

 is often found in theJ»ladder of the frog. It attaches 

 itself in its youth to the gills of tadpoles, passes thence 

 through the food canal to the bladder, where it develops 

 slowly for years. 



Gyrodactyhts, found on the gills and fins of fiesh-water 

 fishes. It is viviparous, but the embryo, before it is 

 extruded, itself contains an embryo, and this in turn 

 another, so that three generations of embryos are re- 

 presented simultaneously. 



Diplozoon paradoxum consists of two individuals united. 

 The single embryo (Diporpa) is at first free-swimming, 

 but becomes a parasite on the gills of a minnow, and 

 there two individuals unite very closely and permanently. 



Tristomum, with three suckers, is not uncommon on the 

 skin of some marine fishes. 



B. Trematodes with indirect development — Digenetic. 



e.g. Distomum, with numerous species. 



Bilharzia hizmatobius, a dangerous parasite of man, widely 

 distributed in Africa, e.g. in Egypt. It infests the urinary 

 and abdominal blood vessels, causing inflammation, 



