132 MANUAL OF ZOOLOGY SECT. 



tine, with their branches, thus form a complicated branching 

 system, the ramifications of which extend throughout the 

 whole of the body. There is no anus, or aperture of com- 

 munication between the intestine and the exterior, the only 

 external opening of the alimentary system being through the 

 mouth. 



A branching system of vessels the water-vessels or 

 vessels of the excretory system ramify throughout the body. 

 A longitudinal main trunk opens outwards by means of the 

 excretory pore. In front it gives off four large trunks, each 

 of which branches repeatedly, the branches giving off smaller 

 vessels, and these again still smaller twigs, until we reach a 

 system of extremely fine microscopic vessels, or capillaries. 

 Each of these ends internally in a slight enlargement 

 situated in the interior of a large cell, a flame-cell, with a 

 bunch of vibratile cilia, or a single thick cilium, in the 

 interior. 



The Fluke has a nervous system, the arrangement of 

 which partakes of the bilateral symmetry of the body. The 

 central part of this system consists of a ring of nerve matter, 

 which surrounds the oesophagus, and presents two lateral 

 thickenings or ganglia containing nerve-cells, and a single 

 ganglion situated in the middle line below. From this are 

 given off a number of nerves, of which the chief are a pair 

 of lateral cords running back to the posterior end and 

 giving off numerous branches. There are no organs of 

 special sense. 



The reproductive organs are constructed on the herma- 

 phrodite plan, i.e., both male and female organs occur in the 

 same individual. The male part of the apparatus consists of 

 testes, sperm-ducts or vasa deferentia, and cirrus. The testes 

 (/<?.) are two greatly ramified tubes which occupy the middle 

 part of the body, one situated behind the other. From each 



