STRUCTUEE OF THE KOSTELLUM. 393 



on the anterior root-process, in consequence of which the hooks are 

 respectively retracted or erected. 



The rostellum is in principle very like the proboscis of the 

 EchiTwrhynchi, only that the latter has on the whole a more powerful 

 development, and is able to exhibit much more effective movements. 

 This resemblance is not, however, always equally evident, as a closer 

 examination will show us.^ 



I regard as the simplest form of this rostellum that which is found 

 in Tcenia cucumerina of the dog and cat {= T. elliptica, Auct.). It 

 consists of a closed sac, which, when quiescent, has an oval form, 

 and is longitudinally embedded in the crown of the head in such a 

 way that the anterior segment, which bears the hooks, protrudes like 

 a boss ensheathed by the cuticle. The boundary of the sac is formed 

 of a structureless membrane, which is 

 possessed of considerable elasticity, 

 and is firmly connected with the sub- 

 jacent musculature. The latter is 

 composed of two kinds of fibres — some 

 circular, which surround the posterior 

 two-thirds of the sac, and which lie, 

 therefore, behind the hooks, and some Fig. 229.— Rostellum of Twnia 

 longitudinal, which belong exclusively cucumerim. ( x uo.) 



to the anterior portion, and run from the circular muscles, con- 

 verging towards the apical surface of the sac. The interior of the 

 sac is fiUed with a somewhat soft, clear, connective substance, which 

 is penetrated by a network of fine fibres, and encloses numerous 

 nucleated cells. 



These two groups of muscles are obviously to be regarded as antago- 

 nistic. By the contraction of the circular fibres, the posterior half of 

 the rostellum is constricted, and instead of cylindrical it becomes 

 club-shaped, owing to the accumulation of connective substance in 

 the thereby distended anterior end. The hooks are forced to change 

 their position on the now more strongly curved surface ; if the 

 pressure, as one would expect, be greater on the posterior root- 

 processes, which are directed towards the apex, then the points of the 

 hooks must move backwards and therefore sink in. When the cir- 

 cular muscles are again relaxed, the longitudinal fibres^ empty the 



' As to the structure of the rostellum, see, in addition to Leuckart ( " Blasenband- 

 wurmer," p. 63, note), also especially Nitsche, Zeitsclir. f. wiss. Zool., Bd. xxiii., p. 181, 

 and Steudener, AbhandL. naturf. Gesdlseh. Halle, Bd. xiii., p. 408, 1877. I must, how- 

 ever, note that I am not able entirely to agree with Nitsche's conclusions, especially in 

 thia, that I regard the ' ' elastic cushion " of the cystic tape-worms as muscular, and as 

 in reality forming the rostellum. 



^ In spite of their somewhat divergent insertion, these retractors plainly represent the 



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