232 MANUAL OF ZOOLOGY. 



of many animals. The length of this composite organism 

 varies from less than an inch to several yards. 



Singular as is the composition of the mature Tape-worm, 

 still more extraordinary are the phenomena observed in its 

 development, of which the following is a brief account : 



" Proglottides," or the sexually mature segments of a Tape- 

 worm, are only produced within the alimentary canal of man, 

 or of some other warm-blooded vertebrate. The development 

 of the ova which are contained in the proglottides, cannot, 

 however, be carried out in this situation ; hence the compar- 

 ative harmlessness of this parasite, and hence the name of 

 " solitary worm/' which is sometimes applied to it. For the 

 production of an embryo, it is necessary that the ovum should 

 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, which for some time 

 retain their vitality and possess some power of movement, de- 

 compose, and the ova are liberated (fig. 114, 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 embiyo (fig. ii3,/)is now 

 called a " proscolex," and consists of a minute vesicle, which 

 is provided with three pairs of siliceous spines, fitted for bor- 

 ing 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 pos- 

 terior extremity, when it is called a "scolex" (fig. 114, 2, and 



