ARTHROPODA 



207 



Sense Organs. — Tactile and olfacto-gustatory senses are ex- 

 tremely well developed in the group. ^^ Equilibratory organs 

 (otocysts) are found at the base of the antennules in the Crustacea. 

 In the mosquito we find auditory vibratile hairs, while the grass- 

 hopper has specialized tympani^ with nerve cells for the reception of 

 sound waves. 



Vision is accomplished by two kinds of eyes, simple ones called 

 ocelli^ and compound ones. The compound eye consists of om- 

 matidia arranged radially around the end of the optic nerve. Each 

 ornmatidium consists of an external cornea (facet), a cellular cone- 

 like lens, sensory retinal cells which receive the light, and pigment 

 cells which separate the retinal elements of the ommatidia from each 

 other. 



Fig. 97. Three trilobites. A, a Cambrian species; 5, a Devonian species, showing a 

 compound eye; C, an Ordovician species. (From Norton, Elements of Geology.) 



Fossil Relatives. Eurypterida (Merostomata) .—This order is 

 linked with Limulus by many Paleozoologists. Specimens six feet 

 long are found in strata from the Cambrian to the Carboniferous. 

 The head is small and unsegmented, bearing two lateral compound 

 eyes, two median ocelli, and six pairs of gills, covered by plates. 



" In insects the chemical sense is much more highly developed than in man. The 

 mouth parts and tarsal segments of the red admiral butterfly {Pyrameis atalantd) are 

 256 times as sensitive to saccharose as the human tongue. Olfaction in the bee is keen; 

 but the bitter substances such as quinine are readily accepted when mixed with saccha- 

 rose. (D. E. Minnich. 1929. The chemical senses of insects. Qu. Rev. of Biol., 

 vol. 4, no. I, pp. 1 00- 1 12, Mar.) 



