250 Description of the Plates. 



of different species of Taeniae differ often very little or not at all 

 from each other, even though their ova would give rise to as 

 different a series of phaenomena as are presented by the histories 

 of the Taenia caennrns and the Taenia serrata, the ova from the 

 former of which would produce in a sheep such an organism as that 

 figured at 6 ; whilst the ova of the latter would produce in a sheep 

 no appreciable effect at all. The Medusae of different species of 

 Polypes are similarly much alike ; and in each case it is possible 

 that the similar mode of life which the sexual zooid in each set of 

 cases goes through_, accounts for the loss at that particular stage 

 of its specific and distinctive characteristics. The reverse is the 

 case in the developmental history of the parasitic Crustacea, in 

 which the larvae are alike, and the sexual animals very different 

 in structure. See Van Beneden, l. c, pp. 113, 148; Polypes, 

 Introd., p. 7. 



Figure 5. 



Embryo or proscolex of an ordinary Taenia, armed, as is the case except in certain 

 marine fish-infesting Taeniae {Tdrarhijnchus) with six spines ; after Van Beneden, 

 I. c, pi. xxvi., fig. 27. 



Such an embryo as this would be of about three times the size 

 of a human blood-corpnsclej o-o22-o-028 M?n, and when set free 

 from the hard shell, which is not drawn in this figure, by the action 

 of the digestive fluids of its host, it would bore and push its way 

 from the mucous surface of the intestinal tract into the blood-vessels, 

 and so pass along them into the liver, a very common place for the 

 development of the cystic stage, or, subsequently, into other organs. 

 The two spines of the central pair of the three are symmetrical, and, in 

 piercing the portal radicles, they have an antero-posterior movement 

 analogous to that executed by the oral stylet of certain parasitic 

 Crustaceans. The two spines again on the extreme right and left 

 of the series are symmetrical with each other, as are the two re- 

 maining ones, which, in numbering the whole series from left to 

 right, would stand as 2 and 5. These two latter pairs move in the 

 piercing of the tissues much as the fore limbs do in swimming. 

 The wound made is, on account of the small size of the embryo, 

 readily overlooked ; and it has been incorrectly supposed that the 

 embryos found their way into the liver by the way of the bile- 



