414 G. H. Park 



arker. 



A 



a whole animal, it does so in response to a wave of stimulation 

 from the aboral pole, the cessation of which is followed by the 

 cessation of the movement in the plate. When one plate with a 

 small amount of protoplasm attached continues to beat, as it 

 often will for hours, it does so because the fragmentary condition 

 of its base exposes this part to continual stimulation. I see no 

 reason to assume that the plates possess an autonomy that is 

 inhibited much of the time by the animal. 



A fragment of a swimming-plate of Mnemiopsis made by 

 splitting the plate lengthwise will continue to beat if a small basal 

 mass of protoplasm is still attached to it. Whole swimming- 

 plates or fragments of plates cease to beat when the base is trimmed 

 off to such an extent that only the swimming-plate proper is left. 

 In this respect the plates ot Mnemiopsis resemble those of the 

 ctenophores on which Verworn ('90, pp. 158, 161) experimented. 

 This failure of the isolated plates to vibrate has generally been 

 attributed to the loss of a stimulus normally received from the 

 basal protoplasm, but Putter ('03, p. 42) has suggested that it 

 may be due to the rapid death of the plates- after isolation from 

 the living substance of the animal. That this is not so in Mnemi- 

 opsis is seen from the fact that a fragment of a plate cut off from 

 its basal protoplasm and kept in sea water half an hour trembles 

 and curves when a little picric acid is applied to it just as the 

 living plates do on a whole animal when this reagent is poured on 

 them. I therefore believe that the quiescence of isolated plates is 

 due to the absence of a stimulus to contraction and not to early 

 death. 



It is evident from what has preceded that the rows of swimming- 

 plates of ctenophores ordinarily beat in pairs corresponding to the 

 quadrants of the animal's body and that the plates of any row 

 beat metachronally beginning ordinarily at the aboral end. As 

 Chun ('80, p. 172) long ago pointed out, that which regulates their 

 beat proceeds usually from the region of the aboral pole and here 

 four centers must be assumed, one for each quadrant of the ani- 

 mal's body. It is also evident that the regulating influence in its 

 passage from the aboral pole is strictly limited to the bands leading 

 from the sense body to the rows of plates, and to the rows of 

 plates themselves, and that, though the waves usually start from 

 the aboral end and progress toward the oral one, they may in 

 some species reverse and run some distance aborally. All these 



