OUR DOMESTICATED ANIMALS. 103 



crown of hooks (a), and three ordinary segments (b, c, d), 

 the lowermost of which is sexually mature, displaying 

 numerous eggs internally. Water vessels traverse the 

 entire length of the worm. 



One of the strangest points connected with this 

 entozoon is the extraordinary provision made for its pro- 

 pagation. In ordinary cases one tapeworm only results 

 from the growth of the products of a single egg ; but 

 here we may have thousands of tapeworms all resulting 

 from a solitary germ. 



I must explain briefly. 



Eggs escape from the dog per anum. One swallowed 

 by any herbivorous animal, say a sheep, will (by a 

 lengthened process of development, the details of which 

 I need not give) eventuate in the formation of hydatids. 

 These hydatids, under favourable circumstances, will by 

 internal budding produce innumerable heads or scolices 

 (Fig. 22, B), each of which displays the tsenia-like crown 

 of hooks (a), the suckers (b), the calcareous particles (c), 

 and a vesicular body (d) . 



When, therefore, a dog is fed on the viscera of a sheep 

 containing perfect hydatids of this description, all the 

 numerous heads become developed into tapeworms in the 

 animal's intestines. This has been proved over and over 

 again by experiment. 



Most of the heads are developed in delicate sacs, termed 

 brood-capsules, one of which is here represented in the 

 collapsed or broken-up state (Fig. 23). It will be further 

 seen that the seven attached heads have their respective 

 crowns of hooks inverted and concealed within the vesi- 

 cular body ; and their appearance in this condition 

 strikingly contrasts with that displayed by the single 

 echinococcus head figured above. 



