ASTERIAS PALLIDA. 



241 



symétrie de la larve et il est tout à fait superflu, au moius dans l'état 

 actuel de nos connaissances, de se baser sur ce dernier pour déterminer 

 le premier" [Cuénot, '91, p. 331]. And yêt, if I am not greatly 

 mistaken, I I)elieve that I can prove a direct connection between the 

 two principal' planes in question ; and 1 may state at the outset 

 that this connection is that of exact coincidence. 



As an important preliminary step to the discussion of this ques- 

 tion, we must make our mind clear as to the method b}' which we are to 

 determine the plane of reference. This is indispensable, as nearly all 

 externally visible organs shift their positions considerably in the transi- 

 tion ffom the larval to the adult form. It is therefore essential that 

 we should take hold of some organs, the relation of which to the plane 

 of the larva is clearly known and which persist until the aboral disc 

 has assumed its final position. Such organs of reference are found in 

 the three brnchiolar arms. One of these arms is dorsal and median, 

 and the other two are ventral and paired, so that the sagittal plane 

 passes through the tip of the dorsal median arm and makes a right 

 angle with the line that connects the tips of the two paired arms. 

 A glance at Plate XIX will show that these brachiolar arms persist up 

 to a stage in which the adult f<3rm is all but ready, keeping all the 

 while, their relative positions. I have taken tliem as my landmark for 

 two reasons : (1) they are almost the only externally visible organs 

 that are common to both the larva and the adult form ; (2) the plane 

 determined by them cuts the body in two equal portions, passing- 

 through the mouth and anus while these are present, and, after their 

 •disappearance, through the places where they w^ere. 



As already known from the works of preceding students, the 

 aboral disc (a convenient term for the ectodermal thickening that is 

 to form the aboral side of the star) is formed on the right side of the 

 larva in the posterior portion of the body (PI. XX, Fig. 17). It is 



