144 MANUAL OF ZOOLOGY. 



with three pairs of siliceous spines, fitted for boring through 

 the tissues of its host. Armed with these, the proscolex 

 perforates the wall of the stomach, and may either penetrate 

 some contiguous organ, or may gain access to some blood- 

 vessel, and be conveyed by the blood to some part of the body, 

 the liver being the one most likely. 



Having by one of these methods reached a suitable resting- 

 place, the proscolex now proceeds to surround itself with a 

 cyst, and to develop a vesicle, containing fluid, from its 

 posterior extremity, when it is called a 'scolex' (fig. 39, 2). 

 In some of the Tceniada the scolices arc called ' hydatids,' and 

 it is these, also, which constituted the old order of the ' Cystic 

 Worms.' When thus encysted within the tissues of an animal, 

 the * scolex ' consists simply of a taenioid head, with a circlet 

 of booklets and four ' oscula ' or suckers, united by a con- 

 tracted neck to a vesicular body. It contains no reproductive 

 organs, or, indeed, organs of any kind, and can not attain any 

 farther stage of development, unless it be swallowed and be 

 taken for the second time into the alimentary canal of a warm- 

 blooded vertebrate. It may increase, and produce fresh 

 scolices ; but this takes place simply by a process of gemma- 

 tion. In some cases, however, a very partial and limited 

 development does actually take place in the scolex prior to 

 this change of abode, but this is an exceptional occurrence. 

 In these cases the ' neck' of the scolex becomes partially seg- 

 mented, so that it comes to resemble an imperfectly developed 

 Tcenia, and is called a ' strobila-embryo.' The series of changes, 

 however, whereby the scolex is converted into the ' strobila,' 

 or adult tape-worm, can not be carried out unless the scolex 

 gain access to the alimentary canal of a warm-blooded verte- 

 brate. In this case, the scolex attaches itself to the mucous 

 membrane of the intestinal tube by means of its cephalic 

 booklets (when these are present) and suckers. The caudal 

 vesicle now drops off, and the scolex is thus converted into the 

 ' head ' of the tape- worm. Gemmation then commences from 

 its posterior extremity, the first segments being immature. 

 As the first-formed joints, however, are pushed further from 

 the head by the constant intercalation of fresh articulations, 

 they become sexually mature, thus constituting the * pro- 

 glottides ' of the adult Tape-worm with which the cycle began. 

 To the entire organism, with its ' head,' and its mature and 

 immature joints (' proglottides '), the term 'strobila' is now 

 applied. 



In the development, therefore, of the Tape-worm we have to 

 remember the following stages : 



1. The ovum, set free from a generative joint, or proglottis. 



