The next important change consists in the development of a head, which becomes the head of the adult Tænia. This is formed in an involution of the outer wall of the anterior extremity of the cystic worm. This involution forms a papilliform projection 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 laminæ, 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 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 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. 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 cellulosæ_ (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 develop one head, this is not invariably the case. There is a cystic worm found in the brain of the sheep known as _Coenurus cerebralis_--the larva of _Tænia coenurus_, 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?) pole of the cystic worm.