MOULTING OF THE YOUNG BLADDER-WOKM. 345 



appears still earlier, usually in the course of the third week, when the 

 bladder is hardly 1 mm. in size, if so much. Only the bladder-worm 

 of the rabbit attains at this time to a length of more than 2 mm. 

 (cross diameter = 04 mm.), having at an early stage changed from 

 a globular to an extended form. 



The first trace of the head is seen in the subcuticular layer of 

 cells the most important layer in the formative history of these 

 organisms. At a distinct point which we may call the anterior pole, 

 and which is easily recognisable as such in extended forms like Cysti- 

 cercus pisiformis, an active vegetative process sets in. In conse- 

 quence of this, the cellular layer becomes thickened into a meniscus, 

 which soon forms a little protuberance, and grows ever further inwards 

 into the bladder-cavity or the substance which fills it. 



As soon as this has gone on for some little time, a pit-like depres- 

 sion arises on the outer covering of the body, which increases in depth 

 as the protuberance grows, and finally broadens out at its lower portion 

 into a bottle-shaped cavity. The rudiment of the head is thus not 

 solid, but hollow, and its cavity is to be regarded as an invagination 

 from without, especially since the cuticle of the bladder is continued 

 inwards through the outer opening, and completely lines the interior. 



FIG. 186. Young bladder-worms of FIG. 187. Young bladder-worms of T. 



Tcenia saginata. ( x 30.) serrata, with rudiment of head. ( x 12.) 



During the deepening process, the lining envelope is sometimes changed. 

 I have seen bladder-worms in which two or three discharged cuticular 

 membranes lay like conical shells one within the other. At this 

 stage, too, one can directly observe the new formation of the cuticle 

 over the bladder, and this much more distinctly than afterwards, 

 since in these young forms the old skin is usually discharged as a 

 continuous sheath, and remains lying for a while near the body. It 

 is loose and thick, and therefore quite different from the thin new 

 cuticle lying below. 



