V 



PLATYHELMINTHES 



115 



which projects forwards over the brain (end.ant, Figs. 91 and 92), and 

 which has been compared by Lang to the so-called excretory canal 

 of a Ctenophore (v. infra). The origin of the genital organs was 

 not observed by Lang. The characteristic rhatadites appear in the 

 ectoderm cells (rh, Fig. 91) during larval life. 



Fig. 92. — Median sagittal section through a young Polyclade worm {i'ungia aurantiaca) 

 just after its metamorphosis. (A-fter Lang.) 



Letters as in preceding figure. 



THE EVOLUTION OF PLATYHELMINTHES 



Now Lang (1889) was led by the study of the European genera 

 to see a strong resemblance between this type of development and 

 that of a Ctenophore, and he therefore suggested that Polyclada, 

 and, inferentially, all Platyhelminthes, are merely Ctenophora which 

 have become adapted to a creeping form of life. 



In Polyclada as in Ctenophora there are large macromeres which 

 bud off smaller micromeres, and from these last tlie ectoderm is 

 formed. In both groups there is an ectodermal stomodaeum occupy- 

 ing the lower pole of the embryo, and at the upper pole we find the 

 main nervous centre. Further, in both, the primary locomotor organ 

 consists of eight ciliated ridges of ectoderm, and Lang has shown that 

 in Miiller's larva the cilia on the ciliated processes are joined edge- 

 wise so as to form combs. It would perhaps be better to compare the 

 ascending loops of the ciliated band which intervenes between two of 

 the larval processes, to the rib of a Ctenophore. MiiUer's larva would, 

 on this hypothesis, represent the pelagic Ctenophore-like ancestor of 

 Polyclada. 



This hypothesis is by far the most plausible that has yet been 

 put forward to account for the extraordinary Platyhelminthes, and it 

 is strengthened by the reflection that a creeping Ctenophore, Gteno- 

 plana, exists, in which the alimentary canal is merely lobed, and the 

 ctenophoral ribs are very much reduced in length ; and by the fact 

 that a Ehabdocoele is known, Monotus, which still, in the adult con- 

 dition, carries an otocyst above the ganglion, as do Ctenophores. 



