ANIMAL PARASITES. 37 



together, and from supposing that there is anything more than 

 an affinity of form between them. Even the proglottides of 

 the Bothriocephali, which are, perhaps, still more similar to the 

 Trematoda in their form, cannot constitute an exception to this 

 rule. If we would regard the Cestodea and Trematoda as 

 creatures standing in an intimate developmental connection, we 

 must finally inquire also from what species of cestode worm a 

 certain trematode worm is derived ? What cestode proglottis 

 have we before us in any particular trematode worm ? With 

 those Trematoda which dwell in the blood (Distoma Immatobium, 

 Bilharz), it would be difficult to discover what mature cestode 

 proglottis had been able to penetrate into the vascular system 

 of the human subject, and there become transformed into a 

 Distoma. All that we can regard as justifiable in this case is a 

 union of the Trematoda and Cestodea as two independent sub- 

 classes in the class of the flat worms (PLATYELMIA), as we have 

 done in the Introduction. 



After this historical, critical, and systematic summary, I turn 

 to trace the subject in its details. 



I. The mature animal, or the Proglottis. 



From the moment when the hindmost segment or segments 

 of a tape-worm colony have become so far developed as 

 to contain the six-hooked brood ready formed and enclosed 

 in the egg-capsules, this segment seeks to break loose 

 either by itself or in company with several others, in order 

 to continue an independent existence, either in the same 

 place (the intestine of its previous host), or in a different 

 one (in the external world). All this varies according to the 

 species. In Tania, Tetrarhynchus , &c., each joint usually breaks 

 loose; in Bothriocephalus a series of joints. In those cases in 

 which no regular formation of segments occurs, (in Tries- 

 nophoruSj there are merely lateral notches representing the com- 

 mencement of segmentation, in Ligula nothing but groups 

 of hermaphrodite brood-places occur,) no single proglottides or 

 series of segments can break loose, but only single brood-places 

 or fragments of the body of the cestode worm with such brood- 

 places ; or the eggs must escape singly. In the same way, as 

 Moller has strikingly pointed out, in certain polype-stocks the 

 mature individuals separate from the parent, sometimes in a more, 



