STRUCTURE OF PROGLOTTIDES. 87 



In the upper part of each segment a vascular ring is formed, 

 from which smaller vessels issue. The vessels themselves are 

 structureless tubes. According to Wagener, a pulsating tube 

 might be proper to them, but only during the period of their 

 existence in the vesicular state ; this, however, neither Leuckart 

 nor I have seen, and, as always remarked, I rather look for its 

 analogue in the communicating branches of the cephalic vessels. 

 Nevertheless, this vascular system may be an excretory organ. 

 In the Bothriocephali, and in Tanice without a rostellum, the 

 vessels communicate without forming a ring. 



The muscles consist of transverse and longitudinal fibres, 

 which may be best recognised in the neck. Immediately 

 under the skin lies the thick layer of longitudinal fibres. 

 The suckers, peculiarly muscular parts, are with radiate fibres 

 uniting in the middle of the sucker upon which lies a layer 

 of circular fibres, and the rostellum. The latter exhibits a 

 predominance of longitudinal fibres, which at the end of the 

 rostellum often spread themselves in the interior of the body 

 in the form of a tuft. The rostrum itself is filled internally 

 with structureless mass or fluid. Whether small muscular fibres 

 run to the little hooklets of the embryo is still a question in 

 dispute. 



Position of the hooks in the Taniee. — I have already spoken of 

 the position of the hooks in the C} - stic worms, and also of 

 their attachment to the skin, and at the same time remarked 

 that no muscular fibres run to the apices of their shafts. The 

 hooks rather ride upon the skin, and indeed in small impres- 

 sions or pits, which in the large-hooked species, and especially 

 in age, acquire the form of actual sacs, of which, as a matter of 

 course, from the quite different position of the hooks during the 

 vesicular period of existence, we find only slight indications at 

 that time. The elevation and depression or spreading of the 

 hooks are not even now effected by peculiar muscles, but by 

 changes in the movement of the parts of the head, particularly 

 of the rostellum, and especially by the pressure of the tenacious 

 fluid contents of the rostellum to and from the roots of the 

 shafts, and by the rostellum itself being at the same time 

 pushed forward and retracted. The movements cannot pro- 

 perly be compared with simple lever-movements. We may come 

 nearest to the truth if we compare them with the action of those 

 metal hooks made use of by packers in moving bales. But the 



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