516 AUDITORY ORGANS OF THE VERTEBRATA. 



vesicles are closed. They are provided with free otoliths, 

 supported by the cilia of the walls of the sack, but in addition 

 some of the cells of the sack are provided with stiff auditory 

 hairs. 



In many forms these sacks have been observed to originate 

 by an invagination of the epiblast of the foot (Paludina, Nassa, 

 Heteropoda, Limax, Clio, Cephalopoda and Lamellibranchiata). 

 In other instances (some Pteropods, Lymnaeus, &c.) they appear, 

 by a secondary modification in the development, to originate by 

 a differentiation of a solid mass of epiblast. 



According to Fol the otocysts in Gasteropods are formed by 

 cells of the wall of the auditory sacks ; and the same appears to 

 hold good for Cephalopoda (Grenadier) 1 shewing that free otoliths 

 have in these instances originated from otoliths originally placed 

 in cells. 



Crustacea. In the decapodous Crustacea organs, which have been 

 experimentally proved to be true organs of hearing, are usually present on 

 the basal joint of the anterior antenna}. They may have (Hensen, No. 384) 

 the form either of closed or of open sacks, lined by an invagination of the 

 epidermis. They are provided with chitinous auditory hairs and free otoliths. 

 In the case of the open sacks the otoliths appear to be simply stones trans- 

 ported into the interior of the sacks, but in the closed sacks the otoliths, 

 though free, are no doubt developed within the sacks. 



The Schizopods, which, as mentioned in the last chapter, are remarkable 

 as containing a genus (Euphausia) with abnormally situated eyes, distinguish 

 themselves again with reference to their auditory organs, in that another 

 genus (Mysis) is characterized by the presence of a pair of auditory sacks in 

 the inner plates of the tail. These sacks have curved auditory hairs support- 

 ing an otolith at their extremity. 



The development of the auditory organs in the Crustacea has not been 

 investigated. 



& 



The Vertebrata. The Cephalochorda are without organs 

 of hearing, and the auditory organ of the Urochorda is constructed 

 on a special type of its own. The primitive auditory organs of 

 the true Vertebrata have the same fundamental characters as 

 those of the majority of aquatic invertebrate forms. They consist 

 of a vesicle, formed by the invagination of a patch of epiblast, 

 and usually shut off from the exterior, but occasionally (Elasmo- 



1 For the somewhat complicated details as to the development of the auditory 

 sacks of Cephalopods I must refer the reader to Vol. n., pp. 278, 279, and to 

 Grenadier (Vol. I., No. 280). 



