TYPE PLATYHELMINTHES. 161 



be questioned, and there is no reason for regarding a Tcenia for instance 

 as an individual belonging to a higher grade than either of these a view 

 which the first and older theory implies, since it regards a Tcenia as a 

 colony of equivalent individuals. Such a form as Caryophyllceus is an 

 aggregate of individualities of a lower grade, organ-individuals ; and just 

 as the cell-individuals composing these may divide, so the organs, or 

 rather the embryonic masses of cells destined to give rise to them, may 

 bud, producing a reduplication of organs. This reduplication may occur 

 in one or more organs ; in the Accela among the Turbellaria it affects only 

 the testes, in the Alloiocoela it affects both ovaries (the vitellaria being 

 originally parts of the embryonic mass which gives rise to the ovaries) and 

 testes, and in the Rhabdocoela it affects only the ovaries. In the Cestodes 

 the entire reproductive apparatus is reduplicated in this manner, a series 

 being produced, and secondarily a tendency for each member of the 

 series to be capable of separation from its fellows has come about owing to 

 the greater certainty it gives for the perpetuation of the species. A cer- 

 tain amount of individuality of the proglottids is thus brought about, but 

 at the same time the process of strobilation cannot accurately be termed a 

 process of non-sexual reproduction by budding, since the proglottid indi- 

 viduals are not quite of the same grade of morphological individuality as 

 CaryophyllmsSj which the scolex represents. Both views are correct to a 

 certain extent : the strobilation is a budding off of individuals from the 

 scolex, but of individuals of a lower grade ; and the entire strobila is in 

 reality an individual comparable to Caryophyllceus or a Trematode. 



Considering, then, the strobila as a metamere-individual, what are the 

 affinities of the Cestodes ? They seem to have been derived from Trema- 

 todes, the simpler forms without reduplication of the reproductive organs 

 being capable of being regarded either as Trematodes without a digestive 

 tract or as Cestodes without any indications of strobilation. If this be true, 

 indications of their affinity should appear in the life-history in accordance 

 with the biogenetic law. One interesting form deserves mention in this 

 respect Archigetes, which occurs in certain Annelids. It is a Cestode 

 without reduplication of organs and provided with a tail, similar in a gen- 

 eral way to that of a Cercaria. Certain facts in its life-history seem to 

 indicate that Archigetes is comparable, not to an adult Cestode, but to a 

 Cysticercus which has become sexually mature, and it might be expected 

 that similarities to the Trematode Cercaria might be found in Cysticerd. 

 Recently such similarities have been shown to exist in certain Cysticer- 

 coids : a tail-like appendage, which later separates and degenerates, has 

 been described as occurring at this stage of the development ; and further- 

 more it has been suggested that the cavity of the Cysticercoid into which 

 the head is invaginated may be equivalent to the Trematode intestine, later 

 on becoming obliterated by the growth of the parenchyma. The evidence 

 at present available points, then, to a derivation of the Cestode from the 

 Trematodes, and from Trematodes in which the Cercaria-stage had already 

 been established. 



