DEVELOPMENT OF THE TAPEWORMS 301 



We may summarize briefly what has been said regarding these 

 larval forms. We have, firstly, solid larval forms without any bladder. 

 These arise directly from the oncosphere and are of two kinds, 

 plerocercus and plerocercoid. Plerocercus is a solid globular larva with 

 the head invaginated into the posterior portion. Plerocercoid (fig. 208) 

 is a solid elongated larva also with the head invaginated into the 

 posterior portion, which is sometimes very long. Secondly, we 

 have larval forms with bladders from which the scolices arise thus 

 indirectly from the oncosphere. They are of two kinds, cysticercoid 

 and cysticercus. 



Cysticercoid. The bladder is but slightly developed and is usually 

 absorbed again. The anterior portion is, moreover, retracted into 

 the posterior, and in some cases there is a long or a stumpy tail 



(figS. 220, 231). 



Cysticercus, or true bladder worms. (These may be divided into 

 [i] cysticercus proper, consisting of a bladder and one scolex ; 



sec. c. 



c.i/. 



c.v. 



FIG. 198. Diagram of a cysticercoid. Cf. FIG. 199. Diagram of a cysticercus. 



figs. 220, 227. c.v.) caudal vesicle or bladder c.v., caudal vesicle or bladder; i. , in- 



(small) ; sec. c., secondary cavity caused by the vagination of wall of bladder. (Stephens.) 

 growth forward of the hind-body; /.,tail bearing 

 six spines. (Stephens.) 



[2] coenurus, a bladder and many scolices ; [3] echinococcus, a 

 bladder in which daughter bladders or cysts are developed, and then 

 in these multiple scolices.) 



In the case of cysticerci a papilliform invagination forms, pro- 

 jecting into the interior of the bladder (fig. 201). The layer of cells 

 forming the papilla becomes divided into two laminae, the outer 1 of 

 which forms a kind of investing membrane (receptaculum capitis) for 

 the papilla. The head and suckers are now developed on the walls 

 bounding the axial lumen of the papilla. The papilla eventually 



1 I.e., regarded from the interior or centre of the invagination. 



