362 LEWIS PIENRY GOUGH. 



The ventral and dorsal canals are equal in diameter at tlie 

 apex of the scolex, Avhere they botli measure 72 fi ; proceed- 

 ing' away from the apex, or from the scolex, the ventral canal 

 increases in diameter and the dorsal canal decreases. At the 

 base of the scolex the ventral canal measures 128/t in diameter, 

 the dorsal 32 /n. At 10 cm. from the scolex the ventral canal 

 was found to be 32 ju by 80//, the dorsal 16 fi by S2 fx ; at 

 20 cm. the ventral canal measured 96 /<, the dorsal 28 fx by 

 20 fx ; at 30 cm. the ventral had increased to 112 fi by 72 /<, 

 the dorsal decreased to 4//. After this the dorsal canal 

 becomes almost oblitei-ated, whilst the venti-al canal goes on 

 increasing" until it measures 160 jU by 240 /x,, this size being 

 reached at 70 cm. from the scolex. 



The dimensions quoted show that the ventral canal is not 

 always round on section ; its shape varies considerably accord- 

 ing to the state of conti'actiou of the proglottid from round 

 or oval to angular, square or polygonal sections being quite 

 frequent in histologically perfect material. The ventral canal 

 often occupies almost the entire thickness of the medullary 

 substance, leaving scarcely enough space for the genital canals 

 to pass. 



The structure of the excretory canals varies according to 

 the portion of the worm examined. A histological difference 

 can also be observed between the dorsal and ventral canals in 

 the strobila. 



The longitudinal canals possess a fine membrane, which can 

 occasionally be seen to be finely striated at right angles to the 

 long axis of the canal. This membrane is the product of a 

 layer of epithelial cells, which separate it from the parenchyma. 

 These cells are best studied in tlie scolex ; they are, as Bugge 

 (1902) found in Moniezia expansa (Rud.), flat cells; their 

 plasma is collected somewhat more above the nuclei than 

 elsewhere, and their margins are not observable. Seen 

 from the side, one observes that the layer is not equally 

 thick everywhere, being thicker near the nuclei, thinner 

 towards the margins. Exactly the same structure can be 

 observed in Stilesia hepatic a, Wolffhiige], where the 



