PLATYELMINTHES. 215 



A still more complicated form of cystic worm is that known as Echino- 

 coccus, parasitic in the liver, lungs, etc. of man and various domestic Un- 

 gulata. In the adult state it is known as Tcenia cchinococciis and infests 

 the intestine of the dog. The cystic worm developed from the six-hooked 

 embryo has usually a spherical form, and is invested in a very thick cuticle 

 (fig. 96 E and F, and fig. 99). It does not itself directly give rise to Tsenia 

 heads, but after it reaches a certain size there are formed on the inner 

 side of its walls small protuberances, which soon grow out into vesicles 

 connected with the walls of the cyst by narrow stalks (figs. 96 F and 99 C). 

 In the interior of these vesicles a cuticle is developed. It is in these 

 secondary vesicles that the heads originate. According to Leuckart, they 

 either arise as outgrowths of the wall of the vesicle on the inner face 

 of which the armature is developed, which subsequently become involuted 

 and remain attached to the wall of the vesicle by a narrow stalk, or they 

 arise from the first as papillifonn projections into the lumen of the vesicle, 

 on the outer side of which the armature is formed. Recent observers only 

 admit the second of these modes of development. The Echinococcus larva, 

 in addition to giving rise to the above head-producing vesicles, also gives 

 rise by budding to fresh cysts, which resemble in all respects the parent 

 cyst. These cysts may either be detached in the interior (fig. 96 F) of 

 the parent or externally. They appear to spring in most cases from the 

 walls of the parent cyst, but there are some discrepancies between the 

 various accounts of the process. In the cysts of the second generation 

 vesicles are produced in which new heads are formed. As the primitive 

 cyst grows, it naturally becomes more and more complicated, and the num- 

 ber of heads to which one larva may give rise becomes in this way almost 

 unlimited. 



Cysticerci may remain a long time without further develop- 

 ment, and human beings have been known to be infested with 

 an Echinococcus cyst for over thirty years. When however the 

 Cysticercus with its head is fully developed, it is in a condition 

 to be carried into its final host. This takes place by the part of 

 one animal infested with cysticerci becoming eaten by the host 

 in question. In the alimentary canal of the final host the con- 

 nective-tissue capsule is digested, and then the vesicular caudal 

 appendage undergoes the same fate, while the head, with its 

 suckers and hooks, attaches itself to the walls of the intestine. 

 The head and rudimentary trunk, which have been up to this 

 time hollow, now become solid by the deposition of an axial 

 tissue; and the trunk very soon becomes divided into segments, 

 known as proglottides (fig. 99 A). These segments are not 

 formed in the same succession as those of Chaetopods ; the 



