368 ON POLYCERCUS, 



has become developed as an elevation of the proximal wall with the 

 suckers on its sides. The distal end of the rostellum is still quite 

 free ; along the distal edges of the sheath (fig. 2) which invests it 

 are a number of extremely fine spinules, which may have to do 

 with the subsequent establishment of the connection between the 

 rostellum and the distal wall. Opposite the free end of the 

 rostellum is a small rounded aperture perforating the distal wall 

 of the Cysticercoid. 



The next stage observed was that of the fully-formed Cysticer- 

 coid. When this is in the retracted condition (PI. xx. fig. 4) it 

 is an oval body with a depression at the proximal, and a rounded 

 aperture at the distal, end. In the interior is the more or less 

 folded rostellum and the suckers. Running from the distal end 

 of the rostellum to the aperture is a strand, which is the sheath 

 of the rostellum, now become firmly connected with the edges of 

 the opening. The body wall consists of two layers, which are 

 continuous with one another anteriorly. 



Occasionally, especially if the temperature is slightly raised, the 

 rostellum becomes protruded through the aperture (fig. 6), and a 

 continuation of this process results in the complete protrusion of 

 the scolex (fig. 7) — the uninverted outer layer of the wall of the 

 Cysticercoid remaining attached to its proximal (posterior) end as 

 the caudal vesicle, while the inverted inner layer forms the body. 



One of the most remarkable features of this Cysticercoid is the 

 great length of the rostellum, which is nearly equal to the entire 

 leugth of the inverted scolex (-25 to '3 mm.). It is narrow (about 

 •06 mm.) and cylindrical in form, expanded distally to form the 

 expansion to which the hooks are attached. 



The shape of this terminal enlargement varies a little in 

 different individuals ; its breadth averages •! mm, The hooks 

 (fig. 5) are arranged in two alternative circlets situated close 

 together ; they are about forty in number altogether, and their 

 length is '035 mm. Each has two roots, one longer, anterioi-, in 

 line with the free part, the other shorter, internal, nearly at right 

 angles to the main axis. 



