30 CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 



Afterwards the deep, lateral furrows extend upward to the level of the 

 apical sense-organ and the animal acquires the characters of Mnemiopsis. 

 Thus the larvae pass successively through stages characteristic of Pleuro- 

 brachia, then of Bolinopsis, and finally Mnemiopsis. 



This species is common throughout the summer off the southern 

 coast of New England and is also found as far south as the Carolinas. 

 It appears not to occur north of Cape Cod, Massachusetts. 



According to Hunter, 1904, Mnemiopsis leidyi rests at the surface 

 with its mouth upward, and at the bottom of the aquarium with mouth 

 downward. A constant electrical current of from 0.5 to 3 volts causes 

 it to turn the aboral pole toward the anode and to move toward the 

 cathode. It is relatively more resistant to decrease than to increase in 

 temperature of the water. Responses to electrical stimulation under 

 conditions of greater heat than normal show decrease in reaction time 

 up to about 29° C, then rapid increase in reaction time. A slow increase 

 in reaction time occurs below 15° C. 



Parker, 1905, finds that in Mnemiopsis and Pleurobrachia the combs 

 beat metachronally, beginning at the aboral ends of the rows. 



In Mnemiopsis the 2 rows of plates belonging to the same quadrant 

 beat in unison. This is often true in Pleurobrachia, although in this 

 form all 8 rows may beat independently. 



The propagation wave shows scarcely any evidence of reversal in 

 Mnemiopsis, but often reverses in Pleurobrachia. Reversal of the effec- 

 tive stroke of the plates was never observed in Mnemiopsis or Pleuro- 

 brachia. 



A transverse cut across a row of combs in Mnemiopsis isolates the 

 oral part of the row, which, however, soon recovers, although it beats 

 independently of other rows. An isolated plate will beat if it retain a 

 small amount of basal protoplasm. 



Cooling part of a row to 5° C. brings the movements of the cooled 

 plates to a standstill, but does not interrupt transmission of the impulse 

 to beat. Stretching of a part of a row also prevents movement, but 

 does not prevent transmission. The transmission impulse is, therefore, 

 mainly if not wholly nervous in nature, although probably supplemented 

 by mechanical transmission from one comb to its neighbor. 



R. S. Lillie, 1908, finds that in Mnemiopsis mechanical stimulation 

 arrests the automatic activity of the swimming plates. This, however, 

 does not occur if calcium be absent, and the effect decreases as the cal- 

 citmi is decreased. 



Chun, 1880, finds that the litholite concretions of the sense-organs 

 of Ctenophorae are composed of calcitmi phosphate, whereas I find that in 

 Scyphomedusae they consist of calcium oxalate. The functions of the 

 concretions in respect to movement must therefore be quite different in 

 the ctenophorae and scyphomedusae. 



In this connection it is of interest to state that Samassa, 1892 

 (Archiv fur mikroskop. Anat., Bd. 40), finds that the litholites are not 

 cell products, but are contained within epithelial cells which usually 

 retain their nuclei, and are set free from the epithelium and gathered 

 together in a mass at the apical pole. It will be recalled that Verwom, 



