SCOLECIDA. 



119 



One of these i.s placed 



and furnished with two ventral suckers, 

 anteriorly (fig. 74, a a), and is perforated 

 by the aperture of the mouth. The hin- 

 der sucker {ap) is imperforate, and is the 

 main organ of adhesion. Between the 

 two suckers are the openings of the male 

 and female organs of reproduction (c). 

 The mouth opens into a short gullet, 

 which divides into two primary branches 

 which are continued backwards. In 

 Distoma lanceolatum, the divisions of the 

 intestine are simple (tig. 74, i), but in 

 D. hepaticum they are furnished with 

 numerous secondary lateral branches. 

 Dorsally is placed a more or less complex 

 system of water-vessels, which open pos- 

 teriorly by a caudal pore (fig. 74, p), 

 and are filled with a watery fluid. The 

 nervous system consists of a pair of 

 cephalic ganglia, giving off branches 

 backwards. A large portion of the body 

 is further occupied by the very com- 

 plicated reproductive organs, the animal 

 being hermaphrodite. 



Most Trematode worms pass through 

 a remarkable series of changes, usually 

 accompanied by migrations, in their 

 earlier stages ; but these changes are in 

 many cases only imperfectly known. In 

 the case of the common Liver - fluke 

 (Distoma hepaticum), the eggs are ex- 

 pelled from the intestine of the sheep 

 which is infected by the adult worm, 

 and give exit to a little ciliated embryo, 

 which swims about actively in water. 

 This bores its way ultimately into the 

 tissues of a particular kind of Water- 

 snail {Limncea truncatula), where it be- 

 comes encysted. It then passes through 

 a series of extraordinary changes, which 

 cannot be described here, the ultimate 

 result of which is the production of a 

 number of little tadpole-like embryos, known as " cercarise." These 

 escape from the body of the snail, and swim about actively for a 



Fig. ~i. — A Trematode Worm 

 (Distovm lanceolatum), enlarg- 

 ed, a a Anterior sucker, with 

 tlie mouth at its bottom ; ap 

 Posterior sucker ; o Gullet, 

 dividing behind into the two 

 branches of the intestine, 

 which ai'e unbranched, and 

 terminate behind in blind 

 extremities (ii); p External 

 opening of the water-vessels, 

 which divide above so as to 

 cross the blind ends of the in- 

 testine. The remaining letters 

 refer to the different parts of 

 the reproductive organs. 



