278 CTENOPHOR^. Part II. 



way, as already mentioned. Again, the great strength of the oral system determines 

 the very extensive and powerful contractions of the mouth, and, no doul^t, is most 

 active when these animals divide too large a jjrey. 



The slight difference already noticed in the development of the spheromeres is 

 also observable in the extent of the rows of locomotive flappers, and though all 

 terminate at a considerable distance from the abactinal pole, the lateral pairs 

 ajjproach it a little more than the anterior and the posterior pairs; so that the 

 figure circumscribed by their abactinal termination coincides with the outhne of the 

 body as seen from that side {Fig. 3), and overlaps but little the outlmes of the 

 digestive cavity as it apjiears in the same view. On the actinal side the eight 

 rows terminate at the same height, tapering and narrowing gradually as they 

 approach the mouth, and extending nearer to it in proportion as the animal is 

 older. In the largest, the space on the actinal side not occupied by the loco- 

 motive flappers is al^out one sixth of the vertical diameter, and, in young specimens, 

 about one half In the smallest specimens that may be seen with the naked eye 

 the locomotive flappers do not occupy one third of the height. As the animal 

 grows lai'ger, the rows of locomotive flappers not only extend farther and farther 

 toward the mouth, but the amljulacral zones become also more prominent and more 

 distinct, not only owing to the growth of the ovaries and spermaries, but also in 

 consequence of the appearance of a larger and larger number of pigment cells in 

 the epidermal layer. At first these are few, far apart, and rounded in fomi ; but 

 gradually they become more numerous, acquire the stellate or branching, and 

 sometimes highly ramified, appearance (PI. II. Fig. 17) characteristic of ordinary 

 pigment cells, varying in color as well as in form. In younger specimens these 

 cells are of a pale yellowish tint, but become more rosy afterward, and those of 

 the adult assume gi'adually a deeper pink. Now such cells are clustered in larger 

 number upon the spermaries, where they have the deepest color, and, as they 

 extend beyond the locomotive flappers (PI. II. Fig. 17), seem to reach the very 

 margin of the mouth (PI. I. Figs. 1, 2, 4, and 9). 



As in all Ctenophora>, the locomotive flappers consist of combs arranged in rows 

 along the amlnflacral tubes ; but what is peculiar in the genus Idyia is, that the 

 combs taper very suddenly toward the abactinal pole (PI. II. Figs. 14 and 18), 

 while on the actinal side they narrow very gradually, and disappear so insensibly 

 as to be lost like a thread among the pigment cells (PL II. Fig. 17). Owing to 

 the steady and slow motion of this animal, it affords the best ojiportunity to watch 

 the play of the flappers. It may be seen, in Figs. 1, 2, and 3 of PI. I., that the 

 locomotive flappers project beyond the general surface of the sj^herosome, and also 

 that the waves formed jjy the flappers along one and the same row when they 

 move successively may be quite distinct {Fig. 1), while at other times they seem 



