66 ANIMAL PARASITES. 



As soon as the uncini ? are formed, four hemispherical sacs, 

 blind appendicular cavities, make their appearance above them, 

 and on the broadest part of the cephalic process : these are the 

 four sucking discs (bothriu). Internally, they are still at first 

 cjnnected with the central cavity of the cephalic process, and 

 clothed with small deciduous hairs. By the conversion of the 

 parenchyma surrounding them into radiate and annular muscular 

 layers, they become completely shut off, and muscles are also 

 developed round the fifth appendicular cavity, and close it up as 

 a rostral cavity, on which an orifice is still to be found occa- 

 sionally, for a short time, but rarely for a permanency, and which 

 may even become a median sucker on the vertex. 



The formation of the apparatus of hooks, &c, is usually com- 

 pleted towards the sixth week. At the same time calcareous 

 corpuscles collect, and the vessels become developed. We 

 observe four lateral stems, which form a ring round the suckers 

 and rostellum, and give off branches, occurring between the 

 suckers, ascending and descending therefrom, and exhibiting some 

 small variations in the number and arrangements of the ramifica- 

 tions situated towards the suckers. From this time it is easy to 

 cause the protrusion of the head. 



The life of the cestoid embrvo might close with this stage of its 

 development, as the head is so far developed as to be ready at any 

 time for its evolution into a tape-worm. But the opportunity of 

 doing this usually occurs only after a considerable time ; and the 

 cestoid worm employs his long interval to exhaust the rest of its 

 formative faculty and power, in the formation, on the neck and 

 the remains of the embryonic body (caudal vesicle), of the so-called 

 tape-worm body. This consists in a simple elongation of the 

 tubular neck, and if, from the slight extensibility of the recep- 

 taculum capitis, there is no space for this, in a bending to a lateral 

 position, and in a coiling up and conglomeration of the growing 

 body, the histological structure of which cannot be ascertained on 

 account of its richness in calcareous corpuscles ; the length of 

 the body produced is variable. If it becomes so large as to be 

 unable to find room in the caudal vesicle (as in Cysticercus 

 fasciolaris) , the body, with the head, issues entirely from the 

 caudal vesicle. But as through the whole period of cysticercal 

 existence, the hooks are retracted into the head; this is the case 

 also even in Cysticercus fasciolaris. 



From the above metamorphosis, which is principally taken from 



