RADIATE ARRANGEMENT OF THE HEAD. 



277 



differences are simply dependent on age .and development. The an- 

 terior joints are the youngest, and therefore smaller and more im- 

 mature than the posterior, which have gradually attained 

 their full size and maturity. The chain furnishes us 

 with a perfect succession of developmental stages, and 

 consists sometimes of many hundred joints, though 

 sometimes of only a few, in Tcenia echinococcus of three 

 or four (Fi^. 138). 



The differences between head and joints are, mainly, 

 that the former is provided with suckers and hooks, and 

 that the latter enclose highly developed hermaphrodite 

 reproductive organs. But besides this characteristic and 

 essential difference, there are many minor ones, so that in 

 fact hardly any common character can be mentioned. It 

 would be difficult to recognise a flat-worm in the head, 

 since it is usually pear- or club-shaped, and from the per- 

 fect similarity of the dorsal and ventral surface, conforms 

 rather to a radiate than to a bilateral type. This is FIG. 133. 

 most decidedly seen in the Tcenice, where the head Adult specimen 



/T" 1 on\ i ji i i -it P f Twnia echino- 



(tig. 139) is not only constantly provided with four coccus. (xi2.) 

 suckers, arranged two on either side round the longi- 

 tudinal axis, but bears also a neat circle of hooks on the apex, which 

 conform exactly to the architectural relations of the radiate animals, 

 both in their high number and regular arrangement. The interior 

 organs also, and especially the excretory vessels, show a distinct 

 radiate arrangement. In other cases the radiate structure is certainly 

 less striking ; for not only do the hooks often lose their circular ar- 

 rangement, and form groups corresponding in number and position 

 to the four suckers, or even wholly disappear, but even the structure 

 of the suckers changes. Sometimes the four approximate in pairs, 

 and fuse into two more or less simple suckers, thus assuming a bila- 

 teral arrangement ; or, instead of the lateral suckers, two median ones 

 appear, one on the dorsal and one on the ventral surface, as in the 

 genus Bothriocephalus (Fig. 1.40). 



In contrast to this formation of the head, the sexual animals 

 possess an undeniable bilateral structure. This is, however, not 

 always shown with equal clearness and emphasis ; indeed, the lateral 

 symmetry is often disturbed, as also not unfrequently happens in 

 other bilateral animals. But it is only the generative organs which 

 are thus asymmetrical, the others lie on either hand in exactly the 

 same fashion. And sometimes even the generative apparatus is 

 quite symmetrical, as, for example, in the genus Bothriocephalus t whose 

 sexual animals are in a certain sense typical Cestodes. The genital 



