STRUCTURE OF THE ROSTELLUM. 393 



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

 respectively retracted or erected. 



The rostellurn is in principle very like the proboscis of the 

 Echinorhynchi, 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. 1 



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 Tcenia 

 longitudinal, which belong exclusively cumeri, (x HO.) 



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 filled 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 2 empty the 



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

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

 and Steudener, Abhandl. naturf. Gesettsch. 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 

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

 in reality forming the rostellum. 



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



