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 Tcenia and of Bothriocephalus have both of them six 

 booklets, though the head of a developed Tcenia is armed with a 

 coronet of numerous booklets and that of Bothriocephalus is unarmed. 

 These six booklets 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 larvae of Tenrbrio 

 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 Tcenia the name of 8coUx, proposed by VAN BENEDEN, is now 

 appropriated by the common consent of Helminthologists. If now 



1 STEIN, in SIEBOLD and KOELLIKEE'S Zeitschr. f. wissenscliaft. Zool. iv. 1853, 

 s. 407. 



