DISTOMUM. 139 



The excretory system consists of a median duct which 



opens by a pore at the posterior end. It is connected with 



innumerable branches which form a fine net- 



xcretory. ^^^^^ ^ ^^^^ ^y^^ \^q^-s^. Each branch eventually 



terminates in a blind swelling, in the centre of which there 

 depends diflagellum. The flickering motion of these flagella, 

 doubtless causing currents towards the exterior, has given 

 the name q{ flauie-cell excretory organs to the whole system. 



There are no known sense-organs, but the nervous 

 system consists of a ring round the pharynx 



ervous. ^.^^^ ^^^^ lateral ganglia and a small ventral 

 ganglion. From the lateral ganglia are given off two 

 ventro-lateral nerves which pass to the hind end of the 

 body. 



Fig. 71.-- Structure of Distomum. 



Ganglia. 

 Nerve Ring. - 

 Right Lateral ... 



Nerve. 



— f-Left Lateral Nerve. 



y 



Excretory Pore. 

 A, Nervous System. B, Excretory System. 



The cavity of the body between the muscle layers and 

 the alimentary and reproductive organs is almost entirely 

 filled up with a mass of cells, arranged in 

 a mesh-work, to which the name oi parenchyma 

 has been applied. Small cavities between these cells re- 

 present the primitive vascular cavity or hgemocoele. 



The reproductive organs are complex. Distomum is 

 hermaphrodite. The female organs consist of a branched 



Reproductive ^^^.^-^ ^^ ^^^ ^^^^ ^^^^ °^ ^^^ animal, from 



which there passes an ovarian duct. Two 



large paired yolk-glands lie laterally and their vitelline ducts 



