46 



bulletin: museum of comparative ZOOLOGY. 



the number and general distribution of the non-directives is indicated in 

 the following tabulation. 1 



Specimen 



Left Directive Pair 



Intervening Non-directive Pairs 

 Right Directive Pair .... 



Non-directive Pairs 



Additional Directive Pair . . 

 Non-directive Pairs 



1 



16 



G 



1 



3 



1 



13 



H 

 1 



1G 

 

 



= 62 



= 12 



Total Non-directive Pairs 



107 



The numbers of non-directives in the animals with two mouths bear 

 an important relation to those of animals with single mouths. In an 

 enumeration of the mesenteries in the latter, made some time ago 

 (Parker, H J7), I found that in 131 specimens the pairs of non-directives 

 were never fewer than three and never more than fourteen, and that the 

 average was about five and a half pairs (5.G+) for each individual. In 

 the nine double-mouthed specimens the non-directives were never fewer 

 than eight pairs, nor more than nineteen, and the average was a little 

 less than twelve (11.9 — ) for each individual. It thus appears that the 

 double-mouthed specimens have almost exactly twice as many non-direc- 

 tive mesenteries as the single-mouthed ones. 



As can be seen from the tabulation already given, as well as from the 

 figures, the non-directives are unevenly but characteristically distributed, 

 there being usually a large group on one side and a small group on the 

 other side of the pairs of directives, called rights and lefts. Even in speci- 

 mens B, F, and H (Figs. 7, 11, and 13), where the directives are more 

 or fewer than two pairs, this same peculiarity in distribution may be said 

 to exist. In another respect the distribution of the non-directives is re- 

 markably regular. In specimen I (Plate III. Fig. 14) a pair of non-direc- 



1 In this tahle the term left directive pair is applied to the pairs of directives 

 toward the left in the figures on Flate III., and to those toward the bottom in the 

 figures on Plate II. The corresponding pairs on the other oesophageal tubes are 

 called right directive pairs. Eight and left have in this connection only this simple 

 descriptive significance. 



