TAPE-WORMS OF THE SUB-FA311LY AYITELLIMX.E. 363 



transverse canals present the same structure as the longi- 

 tudinal canals. 



I have not observed any special musculature of either the 

 dorsal or the ventral canals in Avitellina. 



In the strobila certain changes take place in the epithelial 

 layer of the canals. The nuclei of the epithelial cells sink 

 into the surrounding parenchyma, the membrane remaining 

 connected to the epithelial cells by a fibrillar structure standing 

 vertical to the surface of the canal (fig. 30). This arrange- 

 ment is especially noticeable in the dorsal canal, but is also 

 present in the ventral canal. Bugge (1898) figures the same 

 radial arrangement of the fibres without comment. 



Fig. 31 shows the dorsal canal at about 25 cm. from the 

 scolex. The lumen has almost disappeared, being only 1 or 

 2 ^t wide. 'J'he membrane has consequently thickened con- 

 siderably, and is seen to consist of two layers, a thin dark- 

 staining inner and a thick light-staining outer layer. The 

 outer layer is followed by a darker layer with radial structure, 

 whose fibres appear to be continuous with the radial fibres 

 belonging to the epithelial cells; the epithelial cells have sunk 

 considerably deep into the surrounding parenchyma. Their 

 plasma shows a fibrillar structure, arranged radially where in 

 contact with the canal, and less definitely around the nucleus. 

 The plasma stains very lightly with orange CI, but takes 

 eosin deeply. The nucleolus is large, its membrane stains 

 distinctly ; it contains a nucleolus and one or two chromatin 

 bodies. At first the cells appear to lie close to the radial 

 fibres; then they grow in length and their nuclei move further 

 away from the canal ; the portion of the cell surrounding the 

 nucleus becomes rounded or retort-shaped, the plasma 

 becoming less dense as it recedes from the canal. A con- 

 nection by its fibrillte with the canal is always to be made 

 out. In appearance these cells suggest glands, which swell 

 and increase in size as they come into function. Their con- 

 nection with the dorsal canal is always evident ; the radial 

 fibres suri'ounding the canal certainly belong to them, not to 

 the parenchyma. In cells which have reached their full 



