r8 4 THE HUMAN SPECIES 



sparsely ciliated in their interior and with a varying number of 

 concretions are held to form the auditory apparatus of worms. 

 And for this there seems better support, since these vesicles are 

 in many cases undoubtedly connected with nerves. Tunicates 

 and Turbellariea possess these vesicles unpaired ; in the Anne- 

 lids they are paired, and, as a rule, situated on each side of 

 the brain-ganglion. 



Among Echinoderms the Synapta is provided with auditory 

 vesicles arranged in double pairs radiating from the origins of 

 the five radial nerve-trunks. Contrary to expectation, in the 

 higher order of Arthropoda, organs of hearing are only found in 

 a limited number, and are not met with in Myriapods and 

 Arachnids. 



In Crustaceans the auditory vesicles lie for the most part 

 in the basal joints of the inner antennae. When closed they 

 contain otoliths ; if open one finds them crowded with fine 

 grains of sand which have entered from outside ; in either 

 case nerve twigs enters the auditory hairs of the vesicles. 



Ears are but rarely present in insects, at best only in Or- 

 thoptera, where they consist of a tympanic membrane whose 

 inner aspect rests on a tracheal dilatation with a nerve-ganglion 

 and rod-like nerve-endings similar to those which are found in 

 the somewhat different auditory apparatus of beetles and diptera. 



The actual proof of the importance of these otolith-con- 

 taining vesicles as organs of hearing is furnished by the Mol- 

 luscs. The highest molluscs, the Cephalopods, possess, in 

 addition to their membranous or cartilaginous ear-labyrinth, 

 vesicles containing nerves and concretions, as, for instance, in 

 the Medusae ; so it may be concluded that this structure is an 

 indispensable part of the auditory apparatus in Invertebrates. 



While tracing the evolution of the organs of hearing in 

 the Vertebrates, we find in the early embryonic stages a fold 

 forming in the integument on either side of the hind-brain at 

 its summit. The auditory vesicle thus formed is at first open 

 externally and is connected with the end-apparatus of the 

 eighth or auditory nerve ; by degrees it becomes closed in 

 and covered by the posterior part of the cartilaginous skull- 

 cap. From the auditory vesicle is developed the membranous 

 labyrinth, whilst the cartilaginous and bony labyrinth are de- 



