348 THE DEVELOPMENT OF CESTODES. 



so-called Cysticercus cellulosce, is almost the only instance of another 

 state of affairs. There the receptacle is separate from the head, and 

 becomes a sac-like envelope, wholly free except at a restricted region 

 posteriorly. This peculiarity is connected with the fact that the head 

 of this bladder- worm, instead of growing in proportion to the receptacle, 

 as is usually the case for a lengthened period, becomes rapidly and 

 considerably elongated, and is at an early stage bent like a knee inside 

 the sac (Fig. 188, C). 



The head remains in this state till it acquires a length of perhaps 

 a millimetre. This growth is accompanied only by histological 

 changes. These are most striking in the neighbourhood of the cuticle 

 which clothes the interior of the head, and which afterwards becomes 

 the cuticular covering of the head of the tape -worm. Here, at 

 a position which corresponds to the subcuticular sheath, the 

 hitherto indifferent cellular mass assumes very early a radiate struc- 

 ture (Fig. 188, A). This is brought about by the cells growing out 

 into a layer of closely appressed radiate fibres. These are the struc- 

 tures which we described in the tape-worm (p. 288) as subcuticular 

 fibre-cells. At first sight one might regard them as epithelial cylin- 

 drical cells. Indeed they seem to have the epithelial structure of the 

 subcuticula; but this appearance is quite deceptive. The close 

 grouping, which makes them look like a cellular layer, is caused by 

 the absence of an interstitial connective tissue. When this is subse- 

 quently developed, one can recognise the truth of our position, and 

 that more clearly in the bladder-worm than in the adult form. To be 

 undeniably convinced of the true nature of the subcuticula, it is 

 sufficient to make a thin section of the swollen pad of connective 

 tissue on the head of a Ccenurus, or on the bladder of Cysticercus pisi- 

 formis, which is further penetrated by an extraordinary number of 

 distinct cross fibres. 



As soon as the subcuticula acquires this radiate structure the adja- 

 cent cells of the head begin their histological metamorphosis. For the 

 most part at least they become extended, and grow into fibres, which, 

 in spite of the still distinct nucleus, are undeniably muscular fibres. 

 The course and direction of the fibres are at first indistinct, gradu- 

 ally, however, they become better defined, those next the subcuticula 

 assuming a longitudinal direction, those further outwards becoming 

 rather transverse. Of course this applies only to the main bands, for 

 there are many deviations in detail. 



At this time the rudiments of the vascular system appear. One 

 can recognise four longitudinal stems which spring at the insertion of 

 the head from the vascular system of the bladder, run downwards to 

 the outside of the longitudinal fibres, and become at the wider end 



