88 HEREDITY AND VARIATION 



or swimming bell — Pseudodytia pentata — a leptomedusan 

 belonging to the family Eucopidae. " It differs from all other 

 Hydromedusae in that it normally possesses 5 radial canals, 

 5 lips, and 5 gonads, all 72° apart, instead of 4 of these 

 various organs 90° apart, as in other Eucopidge." In the 

 structure of its tentacles, otocysts, gonads, and manubrium, 

 in the general shape of its bell, and the arrangement of 

 its tentacles and otocysts, it is so closely similar to Epenthesis 

 folleata, that it seems safe to conclude that the former 

 has been derived from the latter or from some closely allied 

 species. The two forms are somewhat different in colour and 

 slightly different as to the position of the gonads, but the 

 resemblance is exceedingly close, and no one can suppose that 

 a medusoid with 5 radial canals is a primitive form. As there 

 are pentamerous variants of Epenthesis folleata and tetramerous 

 variants of Pseudodytia pentata, we are not aware of any case 

 which more cogently suggests the evolutionary interpretation. 

 As Mayer says, " P. pentata may be called ' a new race ' in the 

 sense that it is evidently derived from Epenthesis, and departs 

 from the quadratic arrangement of organs, which is almost uni- 

 versal among Hydromedusae. It is remarkably variable, and 

 its great commonness attests to its successfulness in the struggle 

 for existence" (Mayer, 1901, p. 20), 



To obviate misunderstanding, it may be observed that by the 

 term " newly arisen " which Mayer uses in reference to Pseudodytia 

 pentata, he means simply that " it has departed widely from the 

 fundamental type of all other Hydromedusae, and that it is appar- 

 ently derived from a genus {Epenthesis) which is itself quite highly 

 differentiated. It is, therefore, ' new ' in the sense that it cannot 

 be a primitive form, although we have no means of determining 

 how long a time it may have been in existence " (Mayer, 1901, 

 p. 8). 



While we cannot exactly demonstrate that Pseudodytia pentata 

 arose by discontinuous variation from Epenthesis folleata, or some 

 closely alHed form, the evidence in favour of that interpretation 



