ARTHROPODA 



FIG. 127. 



of grains of sand. Recent investigations, however, show that 

 the function of this organ is not hearing, so much as that of in- 

 forming the animal of its relation to the pull exerted by gravity, 

 thus enabling it to keep its equilibrium. 



There are two classes of eyes in the group: (a)' compound 

 eyes, made up of numerous similar elements, as 

 in the insects and Crustacea, and (b) simple 

 eyes ocelli found alone in spiders and in many 

 larvae, or in connection with the compound eyes, 

 as in many insects. 



The compound eye is made up of elements 

 (ommatidia) radially arranged about the end of 

 the optic nerve. Each ommatidium is probably 

 capable of forming an image of a limited portion of 

 the field, and consists of (i) a cuticular cornea, 

 appearing externally as a "facet," (2) a cellular 

 lens or cone which directs the rays of light, (3) 

 sensory retinal cells which receive the light, and 



FIG. 127. An ommatidium or eye-element from the eye of the Lobster 

 (after G. H. Parker), c, cornea (cuticle); c.h., corneal hypodermis, which 

 secretes the cuticle; co., cone cells; cr., crystalline cone; n, nuclei; n./., nerve 

 fibres; r.d., distal or outer retinula cells; r.p., proximal or inner retinula cells; 

 rh., rhabdome. 



Questions on the figure. Identify the following regions: 



(1) protecting part including the cornea and hypodermal cells; 



(2) focusing portion, the crystalline cone and the cone cells; 



(3) the pigmental elements of the retina (distal and proximal 

 retinular cells), the former of which prevent rays of light en- 

 tering one ommatidium from passing obliquely into adjacent 

 ones; the proximal cells may be more immediately connected 

 with (4) the nervous elements which unite the eye with the 

 nerve centres. Define an ommatidium. Is it known whether 

 the image is inverted in such an eye as this? 



(4) pigment cells which separate the retinal elements of adjacent 

 ommatidia, and play an important, though not fully understood, 

 r61e in vision (see Figs. 44 and 127). 



326. Library Exercise. If time allows some student might be required to make 

 a more detailed report of the structure of the compound eye in Arthropods and its 

 method of image formation. Other reports may be made, in which drawings of 

 the various sense-organs in arthropods are presented to the class, especially the 

 various types of auditory organs. 



