2I 4 



CESTODA. 



moulded into a Cestode head, which however is developed in 

 an inverted position. The suckers and hooks 

 (when present) of the head are developed on a 

 surface bounding the axial lumen of the papilla, 

 which is the true morphological outer surface, 

 while the apparent outer surface of the papilla 

 is that which eventually forms the interior of 

 the (at first) hollow head. Before the external 

 armature of the head has become established, 

 four longitudinal excretory vessels, continuous 

 with those in the body of the cystic worm, make 

 their appearance. They are united by a circular 

 vessel at the apex of the head. The develop- 

 ment is by no means completed with the simple 

 growth of the head, but the whole inverted papilla continues to 

 grow in length, and gives rise to what afterwards becomes part 

 of the trunk. The whole papilla eventually becomes everted, 

 and then the cystic worm takes the form (fig. 97) of a head and 

 un segmented trunk with a vesicle the body of the cystic worm 

 attached behind. The whole larva is known as a Cysticercus. 

 The term scolex, which is also sometimes employed, may be 

 conveniently retained for the head and trunk only. The head 

 differs mainly from that of the adult in being hollow. 



d 



FIG. 97. CYS- 

 TICERCUS CELLU- 

 LOSE. (From Ge- 

 genbaur, after von 

 Siebold.) 



a. Caudal ve- 

 sicle, c. Anterior 

 part of body. d. 

 head. 



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 

 limacis ; it is medium-sized in Cysticercus 

 celluloses (fig. 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 Cce)iurus cerebralisfae larva 

 of Tcenia ccenunis, 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 hun- 

 dred heads are developed (fig. 96 D). They 

 are arranged in groups at one (the anterior?) pole of the cystic worm. 



FIG. 98. CYSTICERCUS 

 WITH SMALL CAUDAL VESICLE. 



A. Head involuted. B. 

 Head everted. 



a. Scolex. l>. caudal vesicle. 

 c. (in A) six embryonic hooks. 



