SPECIAL ORGANS. 97 



With the Invertebrata the peculiarities noticed in different 

 animals are due to the position of the sac, as well as to the 

 number and color of its otolithes. The motility of the latter 

 is thought to be due to presence of cilia within the capsule. 

 The organ is wanting below Radiata. 



RADIATA. In Medusa the ears are numerous, and are placed 

 at regular intervals along the margin of the disk, each con- 

 taining one or more reiractile otolithes. In Geryonia the cap- 

 sules are thicker, and contain red granules. 



MOLLUSCA. In Tunicata function of hearing is assigned 

 to a capsule containing calcareous bodies lying beneath the 

 nerve mass. In Lamellibranchiata the car is placed near foot, 

 when present. The capsule is elastic, opaque, tenacious, and 

 contains a circular commonly motile otolithe. In Gastero- 

 poda the bars (two in number) are often situated upon pe- 

 riphery of body at the sides and beneath the oesophageal ring,, 

 less frequently above it. The otolithes, as a rule, are numer- 

 ous, motile, and composed of carbonate of lime. In Ceph- 

 alopoda the ear-chambers (two in number) are lodged in the- 

 lower middle portion of the cephalic cartilage, where they 

 form two more or less large, rounded cavities, separated! 

 by a cartilaginous septum, and without any external com- 

 munication. (Siebold.) Each chamber holds in relation to- 

 its walls, by delicate fibrous threads, an otic capsule con- 

 taining a soft calcareous otolithe. 



ARTICULATA. Annelida, in Torrea, possess distinct audi- 

 tory capsules. In Crustacea and Insecta the organ, as a rule^. 

 is appended to the basal segment of one of the anterior ex- 

 tremities. The capsule may be completely enclosed within-, 

 the calcareous walls of the segment, as in Lucifer; or the an* 

 terior portion of the capsule maybe exposed, as in Crustacea, 

 and Insecta generally. In Homarus (lobster) the membrane- 

 is open, and permits siliceous particles to enter from without 

 to assume the functions of otolithes. They rest upon a nun>- 

 ber of hair-like points fixed to the bottom of the sac. Lit 

 Palcemon anterior membranous portion of sac is fissured; 

 and the single ovoidal otolithe rests upon the extremities of 

 hair-like processes projecting from basal surface of capsule*. 



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