THE STRUCTURE OF THE HEAD. 



433 



only in the last portion of the chain, where the demands for space 

 made by the young brood reach their maximum, and the proglottides 

 gradually prepare for their liberation by the continued contraction of 

 their transverse musculature. 



The Head. Passing to the consideration of the parts of the body 

 in detail, we may first note that the head is comparatively large, and 

 distinctly recognisable by the naked eye. In one of my specimens the 

 head was about 2 mm. broad, by about 1*5 thick. Other specimens, it is 

 true, fall far short of this, and since this smaller size is most marked in 

 the bladder- worms, one of which had a head measuring hardly 1 mm., 

 we may infer that the age of the worm is an important factor in 

 determining the size of the head. 

 The head of Tcenia saginata only 

 reaches its full developement subse- 

 quently during the tape- worm stage. 



But it is not the size merely 

 which varies in the different forms. 

 The form and nature of the neck also 

 vary according to the state of con- 

 traction. Even Bremser notes that 

 the latter is in constant motion, and 

 retains the form it possessed at 

 death. When the worm is suddenly 

 killed when in an active state of 

 vitality, the neck is found shortened 

 and broad, almost as broad in fact as 

 the head, which at first sight looks 

 exactly like the truncated end of the 

 neck (Fig. 246 A). On the other 

 hand, the specimens killed slowly in 

 some more indifferent fluid have a 

 long slender neck, which passes back- 

 wards, gradually broadening into the 

 jointed body, while in front it is 

 sharply separated from the head. The latter appears in such cases as 

 an independent appendage of an almost pear-like shape, somewhat flat 

 in front, with rounded border, and a narrow conical basal portion 

 attached to the neck as to a stalk. The largest part of the head is 

 occupied by the cylindrical suckers which measure 0'8 mm. in diameter. 

 After death these are generally retracted ; while in life they are 

 frequently protruded " like a snail's horns." Their openings are 

 usually greatly narrowed, so that they come to have little more than 

 the third of their equatorial diameter. In the specimens I examined 



2 E 



FIG. 246. Cephalic end of Tcenia sagi- 

 nata in retracted (A), and extended state 

 (B). (x 8.) 



