STRUCTURE OF THE HEAD. 607 



in the larger capsules hardly measures more than 0*004 mm. But in 

 spite of this thinness the capsular membrane exhibits a distinct 

 histological differentiation. In the interior may be perceived a 

 delicate, sharply defined cuticle, which lines the cavity, and on the 

 outside a thin layer of clear granular cells, which correspond to the 

 superficial cell-elements of the true parenchyma of the JSchinococcus, 

 and differ little from it historically. At the point of insertion of 

 the head one can often distinguish a pair of vessels, which run thence 

 into the capsular membrane. They anastomose freely with the vessels 

 of the adjacent heads, and sometimes unite in the neighbourhood of 

 the stalk of the brood-capsule into a true network, the offshoots of 

 which are seen passing through the stalk to the parenchymal layer 

 of the bladder-worm. In the latter they soon disappear from sight. 

 At most, one can only follow them into the vessels of the stalk of an 

 adjoining brood-capsule, with which they unite. On the Echinococcus- 

 bladder itself, the vessels, if present at all, are much less distinctly 

 marked than is the case in other bladder-worms. 



I have never been able to find muscular fibres in the brood- 

 capsules, although they not unfrequently exhibit a powerful peristaltic ! 

 motion, which as a rule originates from the point of attachment. The 

 drop-like bodies which lie on the inner surface of the cellular layer 

 of the parenchyma are also wanting ; but, in spite of this, I have no 

 hesitation in regarding the brood-capsules as a local development of 

 the bladder-wall. Since the separate layers have an exactly opposite 

 position to those of the bladder- wall, they may be regarded in some 

 respects as an invagination of the latter, just as we previously assumed 

 (p. 357) when seeking to establish the morphological relation of the 

 Echinococcus to the other bladder-worms. 



As to the structure of the head itself, it is essentially the same as 

 the head-process in the Cysticercus. First of all a discoidal thickening 

 originates in the wall of the brood-capsule, which quickly rises and 

 grows into a club-shaped process, whose longitudinal axis is perforated 

 by a canal-like continuation of the interior of the brood-capsule. 

 Like the cavity of the latter, the canal has on its inner surface a 

 cuticle, and that a firmer one than was found there. Similarly, in 

 spite of the resemblance in histological structure, the wall of the 

 process is much thicker than that of the capsule. 



Although composed only of cells, the rudimentary head has now a 

 striking contractility. It stretches out, and may then contract to per- 

 haps half its former length ; it curves and swings like a pendulum to 

 right and left, or may even be invaginated into the brood-capsule, so 

 that the cuticle then becomes an outer coating, as in the Echinococcus- 

 heads. This invagination sometimes occurs at an early stage, before 



