PL A TYELMINTHES. 



177 



FIG. 98. CYSTICERCDS WITH 

 SMALL CAUDAL VESICLE. 



A. Head involuted. B. 

 Head everted. 



a. Scolex. b. caudal vesicle. 

 c. (in A) six embryonic hooks. 



There are great variations in the relative size of the head and the 



vesicle of Cysticerci. In some forms the 

 vesicle is very small (fig. 98), e. g. Cysticercus 

 limncis ; it is medium-sized in Cysticercus cellu- 

 losce (tig. 97), and in some forms is much larger. 

 The embryonic hooks, when they persist, are 

 found at the junction of the trunk and the 

 vesicle (fig. 98 A, c). Though the majority 

 of cystic worms only develope one head, this 

 is not invariably the case. There is a cystic 

 worm found in the brain of the sheep known 

 as Gcenurus cerebralis the larva of Twnia 

 ccenurus, parasitic in the intestine of the dog 

 which forms an exception to this rule. 

 There appears, to start with, a tuft of three 

 or four heads, and finally many hundred heads 

 are developed (fig. 96 D). They are arranged 

 in groups at one (the anterior 1) pole of the 

 cystic worm. 



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 echinococcus 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 Taenia 

 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 papilliform 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 development, 

 and human beings have been known to be infested with an Echino- 

 coccus cyst for over thirty years. When however the Cysticercus 

 with its head is fully developed, it is in a condition to be carried into 



B. E. 1 2 



