270 CTENOPHORiE. Part IL 



SECTION III. 



THE GENUS IDYIA AND OTHER TRUE BEROIDS. 



I find it very difficult to trace the natural limits of the genera belonging to 

 the femily of the Beroidi\3 proper. With the exception of Milne-Edwards's illus- 

 trations of Beroe Forskali, all the descriptions and figures of these animals are so 

 imperfect that they aflFord very indiiferent means of comparison; and the circumstance 

 that it is absolutely impossible to preserve si^ecimens of these Acalephs for pro- 

 longed examination after their death, necessarily limits all comparative investigations 

 within very narrow bounds. There is another obstacle to a thorough revision of 

 the family, arising from the fact that most S2:)ecies known have only been observed 

 for a short time, and therefore only in one condition of their natural development. 

 Availing myself of the opportunities I have had of studying for the last three 

 years one species of this fimily in every stage of growth, I am able to state 

 positively that the genus Medea is founded on the pecidiarities of the young before 

 they have reached half their size. Several naturalists have already suspected that 

 the genus Medea coidd not be retained, and that it was based upon the exami- 

 nation of immature sjjecimens. I am able to state with confidence that this is 

 really the case. The genus Medea is characterized by the shortness of its rows 

 of locomotive flappers, which do not extend more than half way from the abactinal 

 side toward the mouth, while in the genus Beroe the ambulacra! rows are said to 

 extend all the way to near the margin of the mouth. Now it may be seen (PI. I.), 

 that, in the smaller specimens of the Idyia of our shore {Fig. 6), the rows of loco- 

 motive flappers approach less closely to the margin of the mouth in proportion as 

 the specimens are younger ; and that, while in the largest {Figs. 1 and 2) they 

 extend comparatively much nearer to the edge of the mouth, in the smallest they 

 are so limited as already to answer to the generic character of Medea. I may add, 

 that, in still younger specimens, the difference is even greater. Indeed, in very 

 young specimens, almost too small to be detected by the naked eye, the locomotive 

 flappers are so little developed as to occupy, on the abactinal side of the body, only 

 one third of its height. There can be no doubt, therefore, that the extent of the 

 rows of locomotive flappers does not constitute a generic character among the Beroids 

 propel-, without the special qualification that their extent is increasing with age. 

 Esclischoltz mentions the great length of the cilia as another generic character of 

 Medea; but this also is only a peculiarity of the earlier periods of growth, all Cte- 

 nophoraj when very young having their rows of locomotive flappers much further 



