PLATHELMINTHES 195 



testinal wall by means of the boring movements of their 

 hooklets. In this way apparently they arrive in the blood- 

 vessels, and are probably carried along by the blood current, 

 finally to take up their permanent abode in various organs, 

 very frequently in the liver, sometimes in the brain, in the 

 musculature, etc. There a vigorous growth soon begins ; 

 this is connected with a simultaneous activity of the sur- 

 rounding tissues, which foi^m a membrane about the intruded 

 foreign body. The latter now casts off its hooks, and on 

 its surface there appears a rather thick cuticula, under- 

 neath which circular and longitudinal muscle fibres are 

 differentiated. Beneath these there follows a cortical 

 layer resembling connective tissue, which differs from the 

 central parenchymatous tissue (Fig. 96 B). The latter soon 

 exhibits spaces, in which an aqueous fluid makes its appear- 

 ance. By the coalescing of these spaces with one another, a 

 large cavity filled with fluid finally arises within the body. 

 Herewith the development of the tapeworm has reached the 

 stage which is known as the Gysticercus, bladder-worm, or 

 hydatid. It has been compared to the sporocyst of the 

 Trematoda, although it presents no particular resemblance 

 to it either in structure or in regard to its further develop- 

 ment.^ 



The excretory system has the same organization in the 

 bladder-worm as in the tapeworm. It is composed of capil- 

 laries which arise in ciliated funnels in the tissues, and 

 discharge into larger stems. The latter unite into the chief 

 trunks, which may fuse to form a short sac at the posterior 

 end and there open to the exterior (G. Wagener, Leuckart).^ 



^ [In many cases the formation of a cavity in the Gysticercus is greatly 

 reduced or becomes entirely suppressed. There are found in the lungs 

 of crows and in the body-cavity of Lacerta vivipara, for example, 

 Cysticerci of this kind (Pietocystis variabilis and P. dytldridium Diesing), 

 the body of which is filled with a continuous connective tissue (Leuckart). 

 Such Gysticercus stages of Cestodes have been designated by the name 

 Plerocerci and Plerocercoids (M. Braun), — by the latter when the scolex 

 is only sHghtly marked off from the bladder. Such, to a certain extent 

 aberrant, bladder-worms are found in the Tajniadae, as well as in the 

 Bothriocephalidae and other Gestodes. — K.] 



'■^ [The Gysticerci with long caudal appendages, which occur in in- 



