FORMATION OF THE HEADS IN BROOD-CAPSULES. 579 



group the heads are developed as before from hollow processes hang- 

 ing down into the cavity of the bladder, but these processes are not 

 seated directly on the wall of the bladder-worm, but on special small 

 brood-capsules, which attain about the size of a pin's head, and are 

 budded off in ever-increasing numbers from the inner surface of the 

 worm. The cuticle of the bladder, which is 

 distinguished by considerable thickness, does 

 not take the least share in these processes. 

 Sooner or later, sometimes only after complete 

 development, the heads become invaginated 

 into the interior of the brood-capsule, and 

 there become solid. The place of insertion 

 becomes changed into a thin stalk, which en- 

 closes the vessels of the tape- worm head. The 

 number of heads becomes continually greater 

 with increasing age, and sometimes amounts to FIG. 314. Brood-cap- 



- . i i j i su l e of Echinococcus with 



twelve or more in a single brood -capsule, retracted head and with two 

 Since the bladder, in contrast to the tape- worm appended buds at different 

 head, has attained a very appreciable size, and 



often bears many thousands of brood-capsules, the total number of 

 heads is, of course, extremely large. The older heads not unfrequently 

 fall from their stalks, and waste away within the brood-capsule, but 

 this does not counterbalance the new growth. The total number of 

 heads always becomes larger with increasing age. In some cases it 

 may, without exaggeration, be estimated at several hundred thousands. 



On the other hand, it must be remarked that the formation of 

 brood-capsules and heads is referred to a later period than is usually 

 the case with the cystic tape-worms. One not unfrequently finds 

 Echinococci the size of a pigeon's egg, which as yet contain no brood, 

 and others which remain sterile throughout life. 



As to the histology of these animals, besides the above-mentioned 

 thickness of the cuticle, we should also note the extremely sparse 

 development of muscular fibres in the wall of the bladder, in con- 

 sequence of which the Echinococci have no power of vigorous move- 

 ment. They are as a rule motionless, so that at first sight one might 

 easily take them for simple water-bladders. And this is all the easier, 

 since they are distinguished from the other bladder-worms by very 

 frequently containing (and especially when occurring in man) a 

 varying number of small water-bladders (the so-called " daughter- 

 bladders" or "secondary hydatids"). We shall afterwards return to 

 these structures, and especially to the discussion of their remarkable 

 origin. Meanwhile we shall only mention that in the last case they 

 are the result of a budding, which is not unfrequently repeated in 



