INTRODUCTION TO THE METAZOA 



79 



Both taste and olfactory organs of the invertebrates are rather 

 simple in their organization. They are especially developed in 

 the insects, where they frequently occur as a sac or depression 

 within which there is a bristle or conelike projection. 



Organs of sight, hearing, and balance are frequently much 

 more highly organized than those organs previously mentioned. 

 The association of pigment with sensory epithelial cells frequently 

 denotes the presence of a retina or some sort of structure for 

 reception of light stimuli. Yet there are some organisms, such as 

 the earthworms, which respond to hght stimulation without hav- 

 ing any specialized sense organs for the reception of light stimuli. 

 Pigment spots near the bases of the tentacles in many jellyfishes 

 are usually thought to have a light-percipient function and in 

 some instances even have lenslike bodies associated with them. 



In the turbellarians, there are eyes in which the sensory 

 epithelium forming the retina has undergone considerable speciali- 

 zation, and in many of the marine annelids highly complicated 

 optic organs are found. The 

 highest development of an 

 invertebrate eye is found in 

 cephalopods, the eyes of which 

 very closely resemble those of 

 vertebrates, though the two 

 seem to have no phylogenetic 

 relationship but apparently 

 have arisen independently of 

 each other. 



Compound eyes are highly 

 characteristic of arthropods. A 

 compound eye is composed of an 

 aggregation of similar elements 

 called ommatidia the number of 

 which is indicated by the facets 

 or rounded or hexagonal markings on the surface of the eye. 

 Each ommatidium is bounded externally by a biconvex cornea 

 beneath which is located a fluid or a sohd conehke lens. Beneath 

 this are one or more chitinous rods termed the rhabdomes. 

 Pigment cells surround each ommatidium and nerve fibers pass 

 off from the base of the rhabdomes. 



The term ocellus is frequently taken as synonymous with 

 a simple eye, but from point of view of structure there are two 



Fig. 48. 



Median ocellus of Acilius. 

 {After Patten) . 



