52 SPECIAL ANATOMY OF THE TERRESTRIAL 



glionic globules. Their interior is filled with a trans- 

 parent fluid, containing numerous otoconites, which vary 

 in size, are oval in form, transparent, composed of con- 

 centric layers of carbonate of lime, and frequently have 

 a small cavity in their centre. During life, and for a 

 short time after the death of the animal, the otoconites 

 are endowed with a peculiar vibratory movement, by 

 which they are disposed to accumulate into a mass in the 

 centre of the auditory vesicle. After the cessation of 

 the movement, they become diffused through the fluid of 

 the vesicle. 



Sight. The eyeball is placed beneath the integu- 

 ment, on the outer side of the constriction which exists 

 between the gangliform swellings at the free extremity 

 of the superior tentacula. The optic nerve is derived 

 from the inferior part of the first gangliform enlargement, 

 is tortuous or undulating, and reaches the eyeball at its 

 posterior part. Its course is frequently indicated by a 

 deposit of pigmentum nigrum. 



The eyeball is globular, and is invested exteriorly by 

 a transparent tunic, corresponding to the sclerotica and 

 cornea. 



The choroidea forms two-thirds of a sphere, and is 

 inflected anteriorly into a sort of depressed disk, perfo- 

 rated in the centre. It consists of a delicate, trans- 

 lucent membrane, with a deposit of a single layer of 

 irregularly rounded, or oval, black pigment cells. The 

 interior of the choroidea contains a clear, consistent vit- 



