MORPHOLOGY OF ANIMALS 



tympanic membrane which serve to collect sound waves and 

 which constitute the external ear (Fig. 27, B). 



(3) Visual Organs are differentiated for the reception of 

 light stimuli. Animals without any trace of eyes are sensitive 

 to light (certain protozoans, annelids, many larvae), and it 

 is therefore certain that protoplasm may be directly stimu- 

 lated by light without the intervention of any special organ. 

 In its simplest form an eye consists of one or a few trans- 

 parent cells partially surrounded by pigment in the form 

 of a cup, so that the light can enter only from one side; the 

 pigment not only absorbs light rays, but it optically isolates 

 the cells within from those without this cup (some jellyfish, 

 turbellarians, annelids). The function of such an eye is prob- 

 ably to determine the direction of light, since it could 

 give no image of luminous objects. A slight advance over 

 this simplest type of eye is found in the cup-shaped eyes of 

 certain mollusks; here certain superficial epithelial cells are 

 infolded to form a cup; some of these cells are deeply pig- 

 mented, while other intermediate cells remain clear and 

 unpigmented. The latter are the sensory cells and are con- 

 nected at their bases with nerve fibres. If this cup-shaped eye 

 becomes infolded still further and its opening grows smaller, 

 it forms a "pin-hole camera" type of eye (Nautilus, 

 Haliotis, Fig. 28, A). 



Vesicular Eyes. If the optic cup finally closes altogether, 

 it forms a vesicular eye such as is present in certain mollusks 

 and annelids. The wall of this vesicle, which is turned toward 

 the epidermis, is transparent and may become thickened 

 to form a lens; the opposite wall of the vesicle is pigmented 

 and is known as the retina. In such an eye the free ends of 



C81 3 



