160 MANUAL OF ZOOLOGY. 



joint into a contractile vesicle. It thus appears that all the 

 joints are organically connected together. Whilst the " head " 

 constitutes the real animal, it, nevertheless, contains no repro- 

 ductive organs, and these are developed in the joints or 

 segments (fig. 46, 4), which are produced from the head 

 posteriorly by budding. After the first joint, each new seg- 

 ment is intercalated between the head and the segment, or 

 segments, already formed ; so that the joints nearest the head 

 are those latest formed, and those furthest from the head are 

 the most mature. Each segment, when mature, contains both 

 male and female organs of generation, and is, therefore, 

 sexually perfect. To such a single segment the term " pro- 

 glottis " is applied, from its resemblance in shape to the tip 

 of the 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 Bothriocephalus latus. These two 

 elements namely, the minute head, with its hooklets and 

 suckers, and the aggregate of the joints, or proglottides 

 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 



