THE GENERAL ANIMAL FUNCTIONS 



83 



the general conditions both of origin and structure appear to be much as described 

 for the statocysts. That is to say, the final sensory cells are ectodermal in origin, 

 but now line a sac deeply imbedded in the tissues of the skull. In some of the 

 lower animals, as insects, there are also external vibratile hairs and vibrating 

 membranes which are auditory (Fig. 42). . 



112. Sight. — There are three distinct facts to be noted with respect to visual 

 sensation in the higher forms of animals: the perception of light, the perception of 

 color (i.e., light of different wave-frequency) and the formation of images of ex- 

 ternal objects. It has already been seen (§21) that protoplasm is sensitive and 

 responsive to light without any special organs. The simplest visual organs found 

 in multicellular animals consist merely of epithelial cells containing pigment in 



Fig. 43. 



Fig. 43. Diagrams showing some of the stages in the increasing complexity of the simple eye 

 in Invertebrates. A, simple pigment spot in epithelium having nerve-endings associated with 

 pigment cells (as in some medusae) ; B. pigment cells in a pit-like depression (as in Patella) ; C, with 

 pin-hole opening and vitreous humor in cavity (as in Trocbus) ; D, completely closed pit, with lens 

 and cornea (as in Triton and many other MoUusks); B, pigment area elevated instead of depressed, 

 lens of thickened cuticula (as in the Medusa, Lizzia)i F, retinal cells more highly magnified, ep., 

 epidermis;/, nerve fibre; I, lens; op, optic nerve; p, pigment cells; r, retina; v.h.. vitreous humor. 



Questions on the figures. — What changes take place in the sensory epithelium 

 in this series of figures? What is gained by such a depression as occurs in B? 

 What purpose is served by the pinhole and the vitreous humor of C? Describe 

 the change from C to D. What is gained? What may be the function of the 

 pigment? Compare texts. In what respects does E differ from the other types? 

 What two types of cells are figured as belonging to each retina? What constitutes 

 the retina? 



which changes are wrought apparently by the action of light (Fig. 43, a). These 

 changes affect the nerve fibres associated with the pigment cells and thus the 

 central nervous organ. Such tyes are capable only of giving knowledge of the 

 intensity or, if properly constructed, of intensity and direction of the light and do 

 not form an image of external objects. There are several types of image-fbrming 



