176 CESTODA. 



in many cases disappear or become impossible to make out. In 

 other forms, e.g. Cysticercus limacis, they remain visible, and then 

 mark the anterior pole of the worm (fig. 98, c.). The central part of 

 the body next becomes transformed into a material composed of clear 

 non-nucleated vesicles. Accompanying these changes the embryo 

 grows rapidly in size; a cuticle is deposited by its outer layer, in 

 which also an external layer of circular muscular fibres and an 

 internal layer of longitudinal fibres become differentiated, and internal 

 to both there is formed a layer of granular cells. 



With the rapid growth of the body a central cavity is formed, 

 which becomes filled with fluid, and the embryo assumes the form of 

 a vesicle. At the same time a system of excretory vessels, some- 

 times opening by a posterior pore, becomes visible in the wall of the 

 vesicle. 



The embryo has now reached a condition in which it is known as 

 a cystic- or bladder-worm, and may be compared in almost every 

 respect with the sporocyst of a Trematode (Huxley). 



The next important change consists in the development of a head, 

 which becomes the head of the adult Tsenia. This is formed in an 

 involution of the outer wall of the anterior extremity of the cystic 

 worm. This involution forms a papilliform projec- 

 tion on the inner surface of the wall of the cystic 

 worm, with an axial cavity opening by a pore on 

 the outer surface. The layer of cells forming the 

 papilla soon becomes divided into two laminae, of 

 which the outer forms a kind of investing membrane 

 for the papilla. The papilla itself now becomes 

 moulded into a Cestode head, which however is 

 CYS - developed in an inverted position. The suckers and 

 g" hooks (when present) of the head are developed on 

 genbaur, after von a surface bounding the axial lumen of the papilla, 

 Siebold.) which is the true morphological outer surface, while 



a. Caudal ve- the apparent outer surface of the papilla is that 

 palt of bod teri which eventually forms the interior of the (at first) 

 head. 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 development 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 

 unsegmented 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. 



