CESTODA. 



255 



The tape-worms found in herbivorous animals are probably 

 derived from the Cysticercoids of Invertebrates,- but the inter- 

 mediate host is generally unknown. The same may be said of 

 the tape-worms of birds, only in this case the intermediate host 

 is more often known. 



In some cases the caudal appendage, which is the homologue of 

 the vesicle of the Cysticerci, is elongated (Fig. 210), and the 

 Cysticercoids then present a considerable resemblance to the Cercaria 

 of a Trematode. If this comparison is a just one, as it probably is, 

 the Cysticercoid must be regarded as a more primitive larval form 

 than the Cysticercus. Moreover Caryophyllaeus (Fig. 212) with its 

 single set of generative organs and unsegmented body is probably 

 the most primitive member of the group, and may be compared to 

 the ordinary sexual Trematode. 



a 



FIG. 209. a, Echinococcus-like Cyst from 

 the body -cavity of the earth-worm 

 (after *E. Metschnikoff), containing 

 three Cysticercoids ; b, Cysticercoid 

 with evaginated head. 



FIG. 210. Cysticercoid of 

 Taenia sinuosa from Gam- 

 marus pulex (after Ha- 

 mann). 



Amphilina (Fig. 213) and Amphiptyches are forms intermediate 

 between the Trematodes and Cestodes, while Archigetes (Fig. 211) 

 is either the most primitive Cestode, or a larval form which has 

 become sexually mature. All the other Cestodes differ from the 

 primitive Caryophyllaeus in the fissive reproduction which the body 

 undergoes in the process of strobilization. The only part of the 

 body which is not reproduced in this asexual increase is the organ 

 of attachment; just as in the Scyphistoma (Fig. 132, g) all the 

 organs of the body participate in the fission by which the epliyrae 

 arise except the stalk of attachment. Finally it must be pointed 

 out, that on this view Ligula is not a primitive, but a highly 



* Verh. d. Peter sburger Naturforsch. , 1868, Zool., p. 263. 



