174 THE HUMAN SPECIES 



tip of the tongue in elephants and in the filiform papillae over 

 the whole anterior third of the human tongue. 



Except in man, the epithelium of these filiform papillae be- 

 comes quite horny, according to the animal's species and the 

 grouping of the various papillae. 



In addition to the touch-corpuscles, there exists other ap- 

 paratus for the reception of tactile stimuli, especially the free 

 nerve endings in the lingual epithelium, which have been de- 

 scribed in a small group of animals (rabbits, porpoise, pigs, 

 horses and hedgehogs) but never in man. 



2. Vision. The organs for the perception of light are the 

 next to appear in the evolutionary scale : for there can be no 

 doubt that the marginal bodies of the Ccelenterata should be 

 considered as such in their primitive form of simple pigment 

 spots, or in higher structures, the pigment granules connected 

 with refracting bodies. Numerous, symmetrically placed pig- 

 ment granules are met with even in the lower worms ( Turbel- 

 laria, Trematoda, Nemertinea, Rotifera), which can be shown 

 to be connected with a nerve centre, for the granules are 

 situated either directly on the brain ganglia, or close to it and 

 attached by a nerve fibre. With a further differentiation of 

 structure the pigment granules ensheath the endings of sensory 

 nerves, forming the so-called " crystal cones," specially modified 

 cells, or groups of cells, endowed with the functions of light- 

 perception or refraction (Turbellaria, Trematoda, Tunicata). 

 Among the Annelids, quite primitive structures are found in 

 the leech, but for the most part actual eyes are present lying 

 in pairs either under the integument of the " brain ganglion," or 

 more commonly along the surface of the skin (Syllidae, Ne- 

 reidae}, or on the gill-tufts of the head or in bilateral symmetry 

 on each metamere. 



The majority of echinoderms have simple aggregations of 

 pigment forming so-called " eye-spots," the star-fish alone hav- 

 ing a true eye at the tip of each tentacle composed of numerous 

 crystalline rods enclosed by pigment sheaths similar to the 

 compound eyes of the articulates and certain of the worms. 



The eyes of the Arthropoda are an improvement upon those 

 in the worm, and are either refracting eyes surrounded with 

 pigment, of simple or compound structure but possessing no 



