14 TROPICAL MEDICINE AND HYGIENE 



which in turn may be branched, or there may be no 

 branching and the intestine is then said to be simple. 

 There is no anus (fig. 2). 



A second muscular ring, a fixative sucker, sometimes 

 known as the acetabulum, is present in most of the 

 families. It has no connection with the intestine. The 

 genital opening may be close to it, but this opening is 

 never in the acetabulum. 



The excretory system consists of a series of tubes join- 

 ing to form an excretory duct opening at the posterior 

 extremity. 



The nervous system consists of a bilobed ganglionic 

 mass above the oesophagus, from which are derived two 

 ventral trunks and two smaller dorsal trunks. All these 

 trunks are connected by transverse branches which 

 anastomose. Eye-spots are present in some species. 



Reproductive Organs. The Fasciolidce and Paramphis- 

 toinidce, the largest groups of 'the trematodes, are herm- 

 aphrodite, male and female organs being present in the 

 same individual. The Schistosomidcc form a separate 

 group, as the male and female organs are in different 

 individuals, and there are several testes. 



The testicles are double in the other trematodes, and 

 there is a single ovary. Both testicles and the ovary 

 may be branched, lobulated, or in a single mass. These 

 differences are of value in the subdivision of the group 

 into genera (figs. 3 and 4, B and C). 



Male Organs. The testicles, whether branched or 

 not, are large. They may be situated posteriorly or 

 anteriorly. From each testicle proceeds a tube the vas 

 deferens which passes forward and unites with its fellow 

 to form a sac the cirrhus pouch in which is enclosed 

 a protrusile organ the penis. The opening is close to 

 the opening of the female organ in the hermaphroditic 

 species (fig. 3). 



Female Organs. These are more complicated. There 

 is a single ovary, round and compact, or branched. 

 From this leads an ovarian tube, and into this tube opens 



