2'8 Introduction to A nimal Morphology. 



and processes connected with it (nerves) permeate the 

 textures. To recognize sound waves a vesicle full of 

 fluid containing a calcareous body (Otolith) is de- 

 veloped, to which passes a nerve from the central 

 system. Such organs of hearing are widely distributed. 



They occur in Medusae, in many Worms in which they are 

 placed usually near the cephalic ganglion, in some Turbellaria, 

 where they are unsymmetrical, sometimes single, while in 

 parasites they are absent. 



In Tunicates there is an unsymmetrical ear vesicle ; in 

 Brachiopods it is only present in the larva ; in Lamellibranchs 

 the vesicles are close to the pedal ; in Cephalophora, to the 

 pharyngeal ganglion ; in Cephalopoda they are cephalic, and 

 either simple vesicles (Octopus), or a more complex labyrinth, 

 both ligamental and cartilaginous, with single or multiple 

 otoliths. 



To appreciate light, we find in Infusoria, &c., 

 brightly-coloured pigment spots or photoscopic eyes, 

 sometimes separate from the nervous system (which 

 may discriminate light from darkness). More definite 

 eyes exist in Annulosa as crystal rods or cones at the 

 end of a nerve surrounded by pigment. A cluster of 

 these may form a single mass (Sagitta, &c.) In higher 

 Arthropods a new element is added, for the chitinous 

 integument over the cones is clear, free from pigment, 

 and transparent, lenslike forming a lenticular cornea. 



Single eyes of this form exist in Corycaea, &c., but com- 

 monly they are in clusters forming compound eyes, as in 

 Insects. In such eyes the optic nerve ends in a granular 

 ganglion, in which the ends of the rods are imbedded ; 

 around these is the pigment mass, and over the end of each 

 is a separate corneal lens. As these are closely clustered, 

 each facet becomes hexagonal. These groups are called 

 facetted eyes, and from 3 to 8,000 may be in each cluster. 

 Sometimes the surface is smooth, and the facetting is only 



