394 



FAMILY T^NIAD^. 



anterior end, and produce a uniform distribution of the connective 

 substance, and by the re-assumption of the former cylindrical shape 

 relieve the root-processes from their pressure. If the muscles act 

 more vigorously, then the arched anterior end of the rostellum may 

 even become more or less deeply depressed, so that any effect on 

 the position of the hooks is removed. 



With the movements of the hooks is usually associated a pro- 

 trusion of the rostellum itself, which is, of course, accomphshed 

 by the surrounding muscular masses. Especially important are the 

 longitudinal body-muscles, which can be traced into the head, and 

 partly attach themselves directly to the outer surface of the rostellum. 

 By these muscles, the latter can be retracted to a variable extent, so 

 that the circular margin of the head becomes closed over it. The pro- 

 trusion is effected by means of the transverse muscles which constrict 

 the body, and in consequence drive what it encloses in the direction of 

 least resistance. In many cases there are special muscular arrange- 

 ments for the purpose, sometimes in the shape of well-developed pro- 

 tractors, which are stretched between the front of the head and the 

 rostellum, sometimes as muscles investing the rostellum laterally and 

 posteriorly, and developed in various ways. Thus, we find in the 

 Tcenia undulata of the thrush, &c., round the proper rostellum, a 

 second muscular sac, which extends nearly to the circle of hooks, thus 

 leaving the anterior end of the bulb free. By means of the longi- 



FiG. 230. — Bostellum of rtETiia «»idufate, after Nitsche. (x 100.) 



tudinal and circular fibres of its walls, it is able to act powerfully 

 on the rostellum, and that all the more since the intermediate space 



retractor muscles running inside the proboscis sheath of Echinorhynchus (see Vol. II-)i 

 which deserve all the more notice since the four proboscides of Tetrarhynchus show exactly 

 the same arrangement of ^Efl^ized by MiCrOSOft® 



