THE STRUCTURE OF THE HEAD. 



521 



found in the size and form of the head. In accordance with the 

 smaller proportions of this worm, its size is considerably less (the 

 average hardly amounting to more than 1 mm.), and its form is more 

 nearly spherical, for the back of the head is more markedly contrasted 

 with the straight part of the neck, and the apex, situated on the 

 rostellum, generally projects like a dome, 0'36 mm. high. The form 

 of the apex is, however, subject to great variations, according to the 

 state of contraction of the rostellum, so that it is sometimes more or 



Fig. 291.— Head of Twnia 

 tolium, ( X 45.) 



FlO, 292. — Apical surface and circle of hooks 

 of Twnia solium, {x 80.) 



less deeply sunk in the wall of the head, and at other times elevated 

 into a conical or plug-like proboscis. After what has been already 

 noted regarding the significance of the rostellum and the motor 

 mechanism of the hooks, it is easy to see that each of these differences 

 in form is associated with a particular attitude. If the apex be re- 

 tracted, the hooks are so placed that their points are directed forwards, 

 pretty much in the longitudinal direction of the body. If the apex be 

 still more deeply sunk, the hooks sometimes come to have their points 

 resting together at a common centre. Thence, by the progressive pro- 

 trusion of the apex, they gradually become directed so far backwards, 

 that the concavity, with the now posterioily directed points, is in 

 contact with the sides of the protruding extended proboscis. 



The size of the suckers corresponds to the smaller development of 

 the head, for they only measure 0-4 to 0-5 mm. But in spite of this 

 they are generally more prominent than in T. saginata. The flattening 

 of the head is, however, less striking, for the sagittal diameter is only 

 a little less than the frontal (Fig. 292). 



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