AUDITORY ORGANS. 425 



the ventral side of the body in close juxtaposition to the pedal 

 ganglia. Except possibly in some Cephalopods, these 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 (Paludma, Nassa, Hdero- 

 poda, Li max, Clio, Cephalopoda and Lamellibranehiata). In other 

 instances (some Pteropods, Lynmoeus, &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 drcapoclous Crustacea organ?, which have been 

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

 the basal joint of the anterior antennae. 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 cliitinous auditory hairs and free 

 otoliths. In the case of the open sacks the otoliths appear to be simply 

 stones transported 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 remark- 

 able 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 supporting 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 hear- 

 ing, 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 (Elasmobrauchii) remaining open. The 

 walls of this vesicle are always much complicated and otoliths of 

 various forms are present in its cavity. To this vesicle accessory 

 structures, derived from the walls of the hyomandibular cleft, are 

 added in the majority of terrestrial Vertebrata. 



The development of the true auditory vesicle will be considered 

 separately from that of the accessory structures derived from the 

 hyomandibular cleft. 



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

 of Cephalopoda I must refer the reader to Vol. i. , pp. '230, 281, and to Grenacher 

 (Vol. i., No. 280). 



