DEVELOPMENT OF THE SUCKEES AND HOOKS. 



351 



distinguished from the other more canal-like portion. It is this lower 

 enlargement which is the seat of all those processes which give the 

 head its characteristic form, and which are at this time (in Cysticercus 

 pisiformis in the course of the fourth week) progressing, so far at least 

 as suckers, hooks, and rostellum are concerned. 



The formation of the suckers is most striking. It is introduced by 

 a change in the form of the inner space, which is produced inwards 

 to form little pockets in the substance of the head- walls; these 

 occur at four points, at equal distances from one another. They 

 become more and more markedly distinct from the rest of the in- 

 ternal space, and represent, of course, the cavities of the suckers. The 

 musculature, which forms such an important part of the apparatus, 

 then arises (Fig. 195) ; the subcuticular sheath, with its radiately 

 arranged cells, surrounds each pocket like a hood, and becomes an 

 independent structure, in which the characteristic arrangement of the 

 muscular fibres very soon appears. The fact that the radiate fibres, 

 which form the greater part of the musculature, arise from the subcu- 

 ticular cells, seems to be in favour of the opinion which we have more 

 than once expressed, that the latter have a closer connection with 

 the muscular than with the epidermal tissue. 



The rostellum originates in a closely analogous fashion. The sub- 

 cuticular tissue at the bottom of the head-cavity (i.e., in the depression 

 between the suckers) which is, as we have seen, occasionally pushed 

 forward into a sort of boss (Fig. 196) forms a cushion (Fig. 194), which 

 is supplied with muscles by the development of the above-mentioned 

 extended cells. 



Fig. 194. 



Fio. 196. 



Fig. 195. 

 Metamorphoses of the head of Cysticercus pisiformis. ( x 45.) 



The hooks arise round about the rostellum, or rather on a small 

 circular ridge ^ which surrounds it (Figs. 194, 195). While the hooks 



^ Later observations force me to modify in some particulars the account I previously 

 gave of the history of the circular ridge (first German edition of this work, Bd. i., 

 p. 245). 



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