408 T. H. JOHNSTON. 



The cortical portion of the parenchyma is comparatively 

 small, the greater part of a transverse section being occu- 

 pied by the medulla and the subcuticular layer. 



Excretory System.— This system consists, on each side, 

 of a very well marked ventral vessel with a lumen of about 

 0*011 mm., whose course is roughly parallel to the margin 

 of the segment, and of a very small dorsal vessel with a 

 diameter of only 0*004 mm. The ventral vessel lies almost 

 in the middle of the lateral part of the medulla, but in the 

 region of the genital pore it becomes displaced ventrally by 

 the genital ducts which pass between it and the dorsal 

 vessel. It may be easily traced forwards through the neck 

 to near the level of the middle of the suckers. In younger 

 segments, its course may be bow-like or even sinuous. The 

 dorsal trunk is situated almost directly above the larger 

 vessel and lies near the cortex. It is not displaced by the 

 genital ducts. At the posterior end of each proglottid the 

 ventral stem gives off a wide transverse or commissural 

 vessel which does not pass straight across from side to side 

 but forms a distinct arch with the convexity facing dorsally. 



Nerves : — The longitudinal nerves are readily seen in 

 stained preparations. They lie ventro-laterally from the 

 ventral vessel, close to the cortex. 



Male genitalia.— There are in each proglottis between 

 25 and 30 testes, each of about 0*05 mm. in diameter. They 

 all lie behind the ovary and occupy the posterior half of 

 the segment, extending from the female glands to the 

 transverse excretory vessel, and from the ventral vessel 

 of one side to that of the other. In transverse section, 

 they are seen to fill nearly all the medulla, but they 

 approach rather more to the dorsal side of it. The vas 

 deferens passes forwards as a strong tube, lying in the 

 median line close to the dorsal boundary of the medulla. 

 It lies well above the vitelline gland, shell gland and other 



