TiENIADJi. 109 



summit in the median line. This prominence, when largely 

 developed, becomes retractile, and when not in use is lodged within 

 a flask-shaped cavity, lined by a sheath and supphed with special 

 muscles ; it is also very frequently armed with a single or double 

 crown of horny chitinous hooks, there being, occasionally, as many 

 as five or six separate circular rows of these organs. Attention to 

 the number, relative size, and disposition of the hooks is often 

 suflacient to determine the particular species. In nearly all cases, 

 the reproductive orifices are situated at or near the margins of the 

 joints, which are bisexual. 



Development. — As a tolerably minute account of the anatomy of 

 the Tseniadse will subsequently appear in my special account of the 

 structure of the common tapeworm infesting man, I at once pro- 

 ceed to select a cestode from the lower animals in order to afford 

 a clear conception of the life-changes and developmental phenomena 

 displayed by this singular group of animals — in common, of course, 

 with the other closely-allied famihes referable to the same order. 



In this view I take one of the six species known to infest the 

 dog, namely, the Tcenia serrata of Goeze. This worm, in its adult 

 state, acquires a length of four feet and upwards, whilst its 

 greatest breadth is scarcely the fourth of an inch. The head is 

 oblong in shape, surmounted by a short and thick rostellum, the 

 latter being surrounded by a double crown of hooks, forty-eight 

 iu number, that is, twenty-four in each row. The mature joints 

 are trapezoidal ; having, however, the inferior angles sufl&ciently 

 prominent to produce, collectively, the characteristic serrated 

 margin of the worm. The reproductive papiUse occur at the 

 centre of the lateral line of each joint, being disposed in an 

 irregularly alternate manner throughout the series. 



When, as Leuckart first pointed out, several of the mature 

 joints (proglottides) are detached from the body of a Tcenia serrata, 

 and are administered to a young rabbit, we do not — upon subse- 

 quently kilhng and dissecting it — detect with the naked eye any 

 structural changes within the body of the animal, although several 



