ANNULOIDA: SCOLECIDA. 161 



ova are liberated (fig. 46, i), when they are found to be 

 covered by a capsule which protects them from all ordinary 

 mechanical, and even chemical, agencies, which might prove 

 injurious to them. In this stage, the embryo is often so far 

 developed within the ovum that its head may be recognised by 

 its possession of three pairs of siliceous hooklets. For further 

 development, it is now necessary that the ovum be swallowed 

 by some warm-blooded vertebrate, and should thus gain access 

 to its alimentary canal. When this takes place, the protec- 

 tive capsule or covering of the microscopically minute ovum is 

 ruptured, either mechanically during mastication, or chemi- 

 cally by the action of the gastric juice ; and the embryo is 

 thus liberated. The liberated embryo is now called a " pro- 

 scolex," and consists of a minute vesicle, which is provided 

 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. 46, 2). 

 In some of the T&niada the scolices are 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 hooklets 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 cannot attain any 

 further 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, 

 nowever, whereby the scolex is converted into the " strobila," 

 or adult tape-worm, cannot 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 



VOL. i. L 



