282 



ZOOLOGY. 



FIG. 125. 



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, ap- 

 pearing externally as a "facet," (2) a cel- 

 lular lens or cone which directs the rays of 

 light, (3) sensory retinal cells which re- 

 ceive the light, and (4) pigment cells which 

 separate the retinal elements of adjacent 

 ommatidia, and play an important, though 

 not fully understood, role in vision (see 

 Figs. 42 and 125). 



FIG. 125. 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.f., 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: (i) protecting part including the cornea 

 and hypodermal cells; (2) focussing 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 

 entering 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? 



319. 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. 



320. Reproduction and the Reproductive Organs. Re- 

 production in Arthropods is sexual. With few exceptions the 

 sexes are permanently separate. There is often much differ- 

 ence in the size, color, structure, and activity of the two sexes. 



