IN THE ANIMAL KINGDOM. 97 



original gemmation. The joint next the head is soon 

 divided by a transverse fissure into two, each of which re- 

 peats the process as soon as it is somewhat grown. Whilst 

 the joints multiply in this way they increase in size in the 

 same proportion, and so, of course, remove the joints from 

 the head.* But at a certain distance from the head, the 

 whole nutritive power is applied to the development of the 

 organs of generation, and this mode of sub-division ceases, 

 though the budding on of new segments from the head con- 

 tinues. The segments are at first very minute, but as their 

 growth now becomes rapid, they soon come greatly to ex- 

 ceed the size of the head from which they were originally 

 derived. It is this enlargement of the segments, when they 

 have been thrust to some distance from the head, that gives 

 rise to the peculiar form of the neck of the tapeworm — 

 attenuated to a mere filament where it joins the head, but 

 thickening behind, as it passes insensibly into the body. 

 Organs of both sexes occur in the same segment, but they 

 do not begin to make their appearance till the joints have 

 acquired considerable dimensions, and are always found 

 more mature as we pass towards the hindmost — ^that is, the 

 first-formed segments of the body. When impregnation 

 has taken place, and the ova are ready for evacuation, the 

 segments break off, and are discharged with the foeces, still 

 retaining a certain degree of contractile vitality, which aids 

 in the dispersion of the contained ova. 



In no species probably of either kingdom are all the three 

 stages before distinguished as protomorphic, orthomorphic, 

 and gamomorphic, more clearly marked than in the Cestoid 

 Entozoa. To the first belongs the contractile six-hooked 

 vesicle discharged from the egg, to the second the Ttenia- 

 head, and to the third the jointed body of the Tapeworm. 

 Of these tliree successive phases, each of the later is derived 



* Eschriclit on the Generation of Intestinal Worms. Ediuburgh 

 Philosophical Journal (Oct., 184'1), p. 340. 



