396 



FAMILY T^NIAD^. 



The main mass consists naturally of fibres, which increase and 

 diminish the internal cavity. The former are by far the most nume- 

 rous and powerful, and consist of radial fibres, which are usually 

 united into bundles and are stretched between the two surfaces of the 

 wall of the cup, and have their outer ends all directed towards the 

 centre of the strongly curved interior cavity (Fig. 234). As in 

 other situations, the fibres are embedded in a clear connective substance, 

 which is on the whole much degenerated, but here and there exhibits 

 distinct nucleated cells. The circular fibres counteract the radial, and 

 lie in groups among the former, preserving on the whole, though with 

 frequent deviations, an equatorial direction. On the border of the cup 

 they are developed into a regular sphincter, which is capable of being 

 greatly narrowed, and then protrudes almost like a diaphragm. At 

 the same time, the radial fibres of the border are also variable, inas- 

 much as they gradually change their former angle of insertion (90°) 

 for one extremely acute. 



Besides these muscles, there is usually present on the convex 

 outer surface a system of meridional fibres, which are much less 

 strongly developed than the others, but in the species with powerful 

 suckers sometimes form an exceedingly beautifully plexus. There are 

 also many other modifications, especially in the superfcial muscula- 

 ture, which have not yet obtained close attention. 



Change in the position of the suckers is, of course, 

 effected by those muscles which are attached to the 

 elastic envelope. They are fibrous bands which are 

 separated from the general body musculature, and 

 mostly belong to the longitudinal system. Some of 

 them run into direct attachment with the external wall 

 of the suckers, while others bend from their former 

 direction, and, crossing the fibres of the adjacent side, 

 enter it at a greater or less angle. In connection with 

 the quadruple number of the suckers, we have pre- 

 viously referred to the fact that the ventral and dorsal 

 surfaces in the Cestodes have but a comparatively 

 slight differentiation, and in the head especially are 

 scarcely distinguishable. These conditions are seldom 

 so pronounced in bilateral animals, but are charac- 

 teristic of those animals which we are wont to oppose 

 directly to them and to call Eadiata. 



It is not uninteresting in this connection to note 



that the head of the TpeniadtB sometimes exhibits a 



of a sexuaUy malformation which we not unfrequently find in radiate 



Tig. 232. — 

 Cephalic end of 

 a Tmnia coenurus 

 with hexamerous 

 symmetry. A, 

 transverse section 



mature 

 (x 25.) 



joint. 



animals 



bwith tetramerous symi 

 igitized byWicroscm® 



metry, viz., an increase of 



