174 CLASS V. 
organs only in the intestinal canal of vertebrate animals: when 
found in other viscera of these animals, or in the interior of inferior 
creatures, they are always immature. The ova, however, of 'Tape- 
worms are never developed in the intestine of the animals which 
harbour the parent worm: still the embryo is so far advanced within 
the ovum contained in mature joints when discharged from the 
intestine that its form may be distinguished. In all instances the 
armature of the embryo is the same, however different it may be in 
the heads of fully developed worms of different species. Thus the 
embryos of Zwnia and of Bothriocephalus have both of them six 
hooklets, though the head of a developed Tenia is armed with a 
coronet of numerous hooklets and that of Bothriocephalus is unarmed. 
These six hooklets are not all of the same form: the pair in the 
middle are not curved at the extremity like the others, they are 
straight, very finely pointed, thinner throughout and also longer 
than the other four, which are also disposed in pairs. The middle 
pair are for penetrating soft tissues, and the rest for helping the 
embryo forward when it has once penetrated them. STEIN’ saw 
these embryos free within the intestinal canal of larves of Tenebrio 
molitor and encysted on the outside of the canal, and justly con- 
cluded that the latter had perforated the canal from the interior, 
having entered by the mouth. The future tape-worm does not 
appear to arise from the embryo by metamorphosis, but to be formed 
within it by gemmation, whilst the six teeth of the embryo are 
rejected when they have performed their office and are found 
dispersed on its outer surface. A bud is seen within the embryo, 
which gradually assumes the special form of the head and neck of 
the future Tape-worm. As the development proceeds the head and 
neck would be permanently enclosed within the embryo in which it 
is being formed, were it not that at the same time a canal from the 
exterior is formed around them, so far as to allow the head and 
neck to be produced when the larva is freed from its cyst. It is 
then found that the neck of the larva is continuous with the body 
of the embryo, which forms a vesicle at its extremity. To this larva 
of a Tenia the name of Scolex, proposed by VAN BENEDEN, is now 
appropriated by the common consent of Helminthologists. If now 
1 STEIN, in StEBOLD and KoxrLuiKEr’s Zeitschr. f. wissenschaft. Zool. tv. 1853, 
8. 407. 
