264 J. E. WODSEDALEK. 



cross section of the head of a may-fly showing the position of 

 Palmen's organ in relation to the other parts, and in his discussion 

 says, "Unter dem Palmen'schen Organ verlauft namlich bei 

 alien 5 von mir untersuchten Ephemeridenspecies ein starker, 

 vom Gehirn kommender Nervenstrang. Seine Lagebeziehungen 

 ergeben sich aus Fig. B, die einen Medianschnitt durch den Kopf 

 einer Ephemera vulgata bei schwacher Vergrosserung darstellt. 

 Der erwahnte Nerv (np) verlauft in der Medianlinie vom Gehirn 

 (g) nach hinten unter dem Palmen'schen Organ (/?) hindurch und 

 heftet sich hinter ihm an der Korperwand an. In einem Theil 

 seines Verlaufs liegt er direct auf dem Nervus recurrens (nr) des 

 unpaaren sympathischen Nervensystems." 



Careful examination of many nymphs showed no evidence of 

 the presence of the two large nerves which Gross speaks of as 

 present in the imaginal species which he examined ; this was also 

 true of the adult specimens which I examined. It appears from 

 his discussion of the subject and from his figure (page 98), that 

 what he speaks of as nerves may possibly be the two muscles 

 which play an important part in the movement of the head. 

 The posterior attachment of these muscles to the exoskeleton 

 evidently corresponds to the attachment of the large nerves he 

 misrepresents . In my preparations very thin sections were made, 

 but no signs of nerves extending directly from the brain to the 

 organ were detected. Taking the structure and function of the 

 organ into consideration we should not expect the presence of 

 such nerves. A mass of rather loose tissue exists between the 

 organ and the brain, and the two are loosely united by means of 

 connective tissue. It is the writer's opinion that the chitinous 

 organ being so loosely supported by the four tracheal tubes 

 exerts a pressure on the surrounding tissues, whereby the dis- 

 turbing stimulus reaches the central nervous system. The ob- 

 servations mentioned on the descent of nymphs in various con- 

 ditions, through the water, particularly the death-feigning and 

 the dead individuals, seems to indicate that the orientation is 

 also, in part, a self-directing process, that is, by the presence of 

 the organ the nymph is swerved into position a matter of 

 physical equilibrium. 



Gross' theory that the organ functions only in the adult speci- 



