ANNULOIDA: SCOLECIDA. 143 



to the tip of tlie tongue. The ovary is a branched tube, which 

 occupies the greater part of the proglottis, and opens, along 

 with the efferent duct of the male organ, at a common papilla, 

 which is perforated by an aperture, termed the 'generative 

 pore.' The position of this pore varies, being placed in the 

 centre of one of the lateral margins of the proglottis in the 

 common Tape- worm (Tcenia solium), but being situated upon 

 the flat surface of the segment in the rarer Botliriocepltalus 

 latus. These two elements, namely the minute head, with its 

 booklets and suckers, and the aggregate of the joints, or pro- 

 glottides, together compose what is commonly called a ' Tape- 

 worm,' such as is found in the alimentary canal of man, and of 

 many animals. The length of this composite organism varies 

 from a few inches 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 compara- 

 tive 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 decompose, and the 

 ova are liberated (Jig. 39, 1), 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 booklets. 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 



