422 Prof. C. Glaus on the Morphology and 



forms left behind at various stages of development, and from 

 a morphological standpoint would have to be regarded as 

 simplified progeny-bearing Cercarias. It follows that the 

 Cercaria alone would figure as the equivalent of Cysticercoid 

 or Cysticercus. Now, as a matter of fact, this larval form 

 does afford an immediate and natural comparison with those 

 developmental forms of the scolex which on a number of 

 other grounds we are compelled to regard as the primary and 

 original ones. These are the little Cysticercoids which 

 inhabit the bodies of Invertebrates, and which have only 

 become known comparatively lately. While authors were 

 formerly inclined to derive the Cysticercoid from a Cysti- 

 cercus, simplified and diminished in size, and so to regard it 

 as a Cysticercus whose vesicle, owing to unfavourable soil, 

 had shrunk up and degenerated into a little appendage 

 scarcely capable of containing the body of the scolex, they 

 will now, on the contrary, have to derive the Cysticercus- 

 vesicle from the enlarged caudal appendage of the Cysticercoid, 

 which has become inflated owing to the collection of an aqueous 

 fluid, and to consider it as a secondary modification which 

 has arisen from this and adapted itself to a parasitic existence 

 in the body of a Vertebrate. That this view is actually the 

 correct one, and that the Cysticercoid and not the Cysticercus 

 represents the primary form, from which the other must be 

 derived, is not only rendered probable at the outset by the 

 simpler structure and smaller size of the former, as well as 

 by its sojourn in the bodies of the phyletically older Inverte- 

 brates, but also by the surprising similarity of form existing 

 between certain Cysticercoids and Cercarias, which renders 

 possible, strengthens, and confirms a direct homology between 

 the two. 



The Cysticercoid of Avion empiricorum, which was first 

 described by Stein, being divided by a constriction into a 

 body and a caudal appendage, at once reminds us of the 

 Trematode-Cercaria. To a much greater extent is this agree- 

 ment seen in Archigetes Sieboldit- this is a Caryophyllid 

 allied to Caryophyllcvus and occurring in Naidaj in a sexual 

 scolex-stage. Leuckart * states that the creature consists 

 " like the Cercaria, of a flattened oval body and a cylindrical 

 tail, which is inserted in a pit-shaped depression at the pos- 

 terior end," so that without closely examining it one would 

 suppose it to be a Cercaria. Not less striking is the resem- 

 blance of the Cysticercoid inhabiting the dog-louse and the 



* Rud. Leuckart,, "Archigetes Sieboldii, eine geschlechtsreife Cesto- 

 denamme, Zeitschrift riir w. Zoologie, Supplementbaiid, 1878, Bd. xxx. 



