I 86 MANUAL OF ZOOLOGY. 



be swallowed by some animal other than the one inhabited by 

 the mature Tape-worm. If this does not take place, the fecun- 

 dated ovum is absolutely unable to develop itself. To secure 

 this, however, the dispersion of the ova is provided for by the 

 expulsion of the ripe proglottides from the bowel, all their 

 contained ova having been previously fertilised. After their 

 discharge from the body, the proglottides decompose, and the 

 ova are liberated (fig. 68, 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 

 protective capsule or covering of the microscopically minute 

 ovum is ruptured, either mechanically during mastication, 

 or chemically by the action of the gastric juice ; and the 

 embryo is thus liberated. The liberated embryo is now called 

 a " proscolex," and consists of a minute vesicle, which is pro- 

 vided with three pairs of siliceous spines, fitted for boring 

 through the tissues of its host. Armed with these, the pro- 

 scolex 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 pos- 

 terior extremity, when it is called a "scolex" (fig. 68, 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 de- 

 velopment does actually take place In the scolex prior to this 

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



