298 BROOKS. [Vol. X. 



each successive stage is adapted for increasing its efficiency as 

 a weight organ, although it is only in the last and most per- 

 fected stage that it affords any basis for the analogy with the 

 "auditory organs" of other animals upon which the Hertwigs 

 base their belief that the sense clubs are ears. 



There is no improbability in the view that the closed vesi- 

 cles, with movable concretions, respond to sound vibrations, 

 and function as auditory organs ; but the view that they 

 performed this function during their incipient stages presents 

 many difficulties, which disappear as soon as we recognize the 

 value of these incipient stages as means for giving to the 

 floating medusa the sensation of space, which is a necessary 

 condition for the perception of an external world and for 

 conscious individuality. 



The Significance of the Marginal Sense-Organs in the 

 Systematic Zoology of the Veiled Medusae. 



The view that the veiled medusae which are set free from 

 hydroid cormi are distinguished from those with direct 

 development by a fundamental difference in the structure 

 of their marginal sense-organs was first put forward by the 

 Hertwigs. 



They say in their memoir on the Nervous System and 

 Sense Organs of the Medusae, p. 153: 



" The marginal sense-organs (* Gehororgane ') are of great 

 interest, not to the morphologist or physiologist alone, but to 

 the systematist as well, for they enable us to separate, on 

 anatomical grounds, groups which were heretofore distinguish- 

 able only through their development. These are the Trachome- 

 dusae \the Hertwigs include in the Trachomedtisae both the 

 Narcomedtisae and the Trachomedtisae of Haeckel^ and the 

 campanularian medusae" {Vesicnlatae, Hertwig). 



"Previous attempts to find any structural distinction between 

 those medusae which develop directly from the o^gg and those 

 which are nurtured on campanularian hydroids have not been 

 successful. Whatever system of organs we consider, we find 

 more diversity within the limits of each group than between 



