546 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



longitudinal muscular fibres are developed in its walls. 

 This bladder- worm (cysticercus), now has its outer 

 wall pushed inwards at the anterior end, and on the 

 involution so formed hooks and suckers become de- 

 veloped, in such a way that when, as next happens, the 

 involution is turned inside out, these hooks and suckers 

 lie on the outer surface of the so-called "head." 



We have now a narrow head and neck with an 

 attached bladder (Fig. 227), the head being at this 

 time hollow, and having in it a 

 circular vessel which communicates 

 with four longitudinal fibres. 



If, during the long time that 

 these " bladder-worms " remain alive, 

 the pig is killed for food, and after- 

 wards insufficiently cooked, they are, 

 when the pork is eaten, conveyed into 



Fig.227. Cyshcercus ,, , J 



cellulose (after Von the human stomach. Here the 



fhfnSd ($f*eX ^adder-like termination becomes ab- 



(c), and Vesicle (a), sorbed, and the neck, increasing in 



length, becomes divided into joints 



which are constantly produced at the anterior end ; 



the oldest joints (proglottids) are, in other words, 



farthest from the head. In them sexual organs are 



developed, and the cycle recommences. 



Distomum hepaticiim, of which several hun- 

 dreds may occupy the liver of one sheep, is of extra- 

 ordinary fecundity, producing at least as many as one 

 hundred thousand ova ; these only pass through their 

 earliest segmentation phases in the warmth of the mam- 

 malian body, but when they escape and reach a 

 moderately warm and moist place, the egg commences 

 to develop rapidly within its firm shell. When ready 

 to escape as an elongated ciliated larva, the embryo 

 bursts the cap of its shell, and begins to move about 

 freely. If the pasture on which it has fallen is moist, 

 the larva soon finds a stream of water along which 



