118 



GENERAL PRINCIPLES OF ZOOLOGY 



animals stagger and lose their balance. Starting from this assumption, 

 recent investigators have attempted to prove that the auditory vesicles of 

 invertebrated animals are exclusively, or at least largely, organs of equili- 

 bration. This would explain the otoliths, for these bodies, of relatively 

 high specific gravity, would affect the crista in different ways according 

 to the position of the body. Statoliths is thus a better name. 



Stimulation by Light is a phenomenon widely distributed among 

 animals and plants; in its simplest form it is manifested by the organisms 



C. 



IV 



FIG. 84. Invertebrate eyes. I, Phyllodoce (an annelid, after Hesse); II, Nau- 

 phanta (annelid, after Greef and Hesse); III, larva of a beetle, Acilius, after Grenacher; 

 IV, a medusa, Lizzia; c, cuticle; d, gland cells which secrete the vitreous body; e, 

 epidermis; C, vitreous body; L, lens; o, optic nerve; oc, ocellus; />, pigment; r, rhabdoms 

 of the retina; s, visual cells. 



collecting in or shunning the lighted spot (positive and negative phototaxis). 

 Phototaxis occurs, even when there are no special organs for the recog- 

 nition of light (Infusoria, Hydra, many worms). It is increased when 

 there are visual cells, that is light percipient spots connected with nerves. 

 These may be on the surface, or deeper in position, if the overlying layers 

 be translucent (earthworms, Amphioxus}. If numerous visual cells be 

 united into a layer this is called a retina. In the lowest developmental 



